drm/i915/fbc: update busy_bits even for GTT and flip flushes

We ignore ORIGIN_GTT because the hardware tracking can recognize GTT
writes and take care of them. We also ignore ORIGIN_FLIP because we
deal with flips without relying on the frontbuffer tracking
infrastructure. On the other hand, a flush is a flush and means we're
good to go, so we need to update busy_bits in order to reflect that,
even if we're not going to do anything else about it.

How to reproduce the bug fixed by this patch:
 - boot SKL up to the desktop environment
 - stop the display manager
 - run any of the igt/kms_frontbuffer_tracking/*fbc*onoff* subtests
 - the tests will fail

The steps above will create the right conditions for us to lose track
of busy_bits. If you, for example, run the full set of FBC tests, the
onoff subtests will succeed.

Also notice that the "bug" is that we'll just keep FBC disabled on
cases where it could be enabled, so it's not something the users can
perceive, it just affects power consumption numbers on properly
configured machines.

Testcase: igt/kms_frontbuffer_tracking/*fbc*onoff* (see above)
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459804638-3588-2-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_fbc.c

@@ -992,13 +992,13 @@ void intel_fbc_flush(struct drm_i915_private *dev_priv,
 	if (!fbc_supported(dev_priv))
 		return;
 
-	if (origin == ORIGIN_GTT || origin == ORIGIN_FLIP)
-		return;
-
 	mutex_lock(&fbc->lock);
 
 	fbc->busy_bits &= ~frontbuffer_bits;
 
+	if (origin == ORIGIN_GTT || origin == ORIGIN_FLIP)
+		goto out;
+
 	if (!fbc->busy_bits && fbc->enabled &&
 	    (frontbuffer_bits & intel_fbc_get_frontbuffer_bit(fbc))) {
 		if (fbc->active)
@@ -1007,6 +1007,7 @@ void intel_fbc_flush(struct drm_i915_private *dev_priv,
 			__intel_fbc_post_update(fbc->crtc);
 	}
 
+out:
 	mutex_unlock(&fbc->lock);
 }