drm/i915: fix psr match conditions screw ups.

Not enough brown paper bags, you'll have to share one.
(oops below).

The initial match condition code was racy (locking is coming I hear).

then along came:
cd234b0b
drm/i915: Do not dereference NULL crtc or fb until after checking

Chris made an attempt to fix it, Ben "reviewed" it.
Daniel merged it.

Then
drm/i915: Make use of intel_fb_obj() (v2)
2ff8fde1

made it worse by removing the obj check later.

All in all, my laptop can't barely turn off the display
without hitting this.

Posted to #intel-gfx out of niceness, but I've merged
this already into drm-next.

Here's an oops.
[   11.528185] BUG: unable to handle kernel NULL pointer dereference at 00000000000000d0
[   11.528233] IP: [<ffffffffa0161fde>] intel_edp_psr_match_conditions+0x1e/0x2e0 [i915]
[   11.528294] PGD 35bc0067 PUD c997c067 PMD 0
[   11.528321] Oops: 0000 [#1] SMP
[   11.528916] CPU: 3 PID: 244 Comm: kworker/3:2 Not tainted 3.16.0-rc4+ #17
[   11.528949] Hardware name: LENOVO 20ARS25701/20ARS25701, BIOS GJET72WW (2.22 ) 02/21/2014
[   11.529004] Workqueue: events intel_edp_psr_work [i915]
[   11.529031] task: ffff8803079fdaa0 ti: ffff8803079c4000 task.ti: ffff8803079c4000
[   11.529067] RIP: 0010:[<ffffffffa0161fde>]  [<ffffffffa0161fde>] intel_edp_psr_match_conditions+0x1e/0x2e0 [i915]
[   11.529129] RSP: 0018:ffff8803079c7d40  EFLAGS: 00010246
[   11.529155] RAX: 0000000000000000 RBX: ffff88030c11c000 RCX: c000000000000000
[   11.529189] RDX: 0000000000000001 RSI: 1df0000000000000 RDI: ffff88030c1190d8
[   11.529222] RBP: ffff8803079c7d60 R08: ffffffff82691140 R09: 0000000000000000
[   11.529256] R10: ffff8803079fdaa0 R11: 3e00000000000000 R12: ffff88030c11c728
[   11.529290] R13: ffff88030c1190d8 R14: ffff88031e2d8e00 R15: 00000000000000c0
[   11.529324] FS:  0000000000000000(0000) GS:ffff88031e2c0000(0000) knlGS:0000000000000000
[   11.529361] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   11.529389] CR2: 00000000000000d0 CR3: 00000000c8d9d000 CR4: 00000000001407e0
[   11.529423] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   11.529457] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   11.529489] Stack:
[   11.529500]  ffff88030c119000 ffff88030c11c728 ffff88030c1190d8 ffff88031e2d8e00
[   11.529541]  ffff8803079c7d88 ffffffffa01679b2 ffff880035b29a80 ffff880307909f00
[   11.529583]  ffff88031e2d4740 ffff8803079c7df8 ffffffff810a78ab ffffffff810a7849
[   11.529624] Call Trace:
[   11.529654]  [<ffffffffa01679b2>] intel_edp_psr_work+0x52/0x90 [i915]
[   11.529689]  [<ffffffff810a78ab>] process_one_work+0x1db/0x540
[   11.529719]  [<ffffffff810a7849>] ? process_one_work+0x179/0x540
[   11.529750]  [<ffffffff810a81ed>] worker_thread+0x11d/0x520
[   11.529779]  [<ffffffff810a80d0>] ? create_and_start_worker+0x60/0x60
[   11.529810]  [<ffffffff810aeb04>] kthread+0xe4/0x100
[   11.529836]  [<ffffffff810aea20>] ? kthread_create_on_node+0x200/0x200
[   11.529870]  [<ffffffff81705ebc>] ret_from_fork+0x7c/0xb0
[   11.529896]  [<ffffffff810aea20>] ? kthread_create_on_node+0x200/0x200
[   11.529926] Code: ba 31 13 f0 c9 85 f6 75 84 eb d0 66 90 0f 1f 44 00 00 55 48 89 e5 41 56 41 55 41 54 53 48 8b 87 68 ff ff ff 48 8b 9f 28 ff ff ff <48> 8b 80 d0 00 00 00 4c 8b 63 28 48 8b 40 48 48 85 c0 0f 84 1a
[   11.530110] RIP  [<ffffffffa0161fde>] intel_edp_psr_match_conditions+0x1e/0x2e0 [i915]
[   11.530163]  RSP <ffff8803079c7d40>
[   11.530180] CR2: 00000000000000d0
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/i915/intel_dp.c

@@ -1767,9 +1767,9 @@ static bool intel_edp_psr_match_conditions(struct intel_dp *intel_dp)
 	struct intel_digital_port *dig_port = dp_to_dig_port(intel_dp);
 	struct drm_device *dev = dig_port->base.base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	struct drm_crtc *crtc = dig_port->base.base.crtc;
-	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
-	struct drm_i915_gem_object *obj = intel_fb_obj(crtc->primary->fb);
+	struct drm_crtc *crtc;
+	struct intel_crtc *intel_crtc;
+	struct drm_i915_gem_object *obj;
 	struct intel_encoder *intel_encoder = &dp_to_dig_port(intel_dp)->base;
 
 	dev_priv->psr.source_ok = false;
@@ -1802,6 +1802,7 @@ static bool intel_edp_psr_match_conditions(struct intel_dp *intel_dp)
 		return false;
 	}
 
+	obj = intel_fb_obj(crtc->primary->fb);
 	if (obj->tiling_mode != I915_TILING_X ||
 	    obj->fence_reg == I915_FENCE_REG_NONE) {
 		DRM_DEBUG_KMS("PSR condition failed: fb not tiled or fenced\n");