drm/i915: don't disable disconnected outputs

This piece of neat lore has been ported painstakingly and bug-for-bug
compatible from the old crtc helper code.

Imo it's utter nonsense.

If you disconnected a cable and before you reconnect it, userspace (or
the kernel) does an set_crtc call, this will result in that connector
getting disabled. Which will result in a nice black screen when
plugging in the cable again.

There's absolutely no reason the kernel does such policy enforcements
- if userspace tries to set up a mode on something disconnected we
might fail loudly (since the dp link training fails), but silently
adjusting the output configuration behind userspace's back is a recipe
for disaster. Specifically I think that this could explain some of our
MI_WAIT hangs around suspend, where userspace issues a scanline wait
on a disable pipe. This mechanisims here could explain how that pipe
got disabled without userspace noticing.

Note that this fixes a NULL deref at BIOS takeover when the firmware
sets up a disconnected output in a clone configuration with a
connected output on the 2nd pipe: When doing the full modeset we don't
have a mode for the 2nd pipe and OOPS. On the first pipe this doesn't
matter, since at boot-up the fbdev helpers will set up the choosen
configuration on that on first. Since this is now the umptenth bug
around handling this imo brain-dead semantics correctly, I think it's
time to kill it and see whether there's any userspace out there which
relies on this.

It also nicely demonstrates that we have a tiny window where DP
hotplug can still kill the driver.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58396
Cc: stable@vger.kernel.org
Tested-by: NPeter Ujfalusi <peter.ujfalusi@gmail.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8133,10 +8133,6 @@ intel_modeset_stage_output_state(struct drm_device *dev,
 			DRM_DEBUG_KMS("encoder changed, full mode switch\n");
 			config->mode_changed = true;
 		}
-
-		/* Disable all disconnected encoders. */
-		if (connector->base.status == connector_status_disconnected)
-			connector->new_encoder = NULL;
 	}
 	/* connector->new_encoder is now updated for all connectors. */