drm/i915: Kill dangerous pending-flip debugging

We can, by virtue of a vblank interrupt firing in the middle of setting
up the unpin work (i.e. after we set the unpin_work field and before we
write to the ringbuffer) enter intel_finish_page_flip() prior to
receiving the pending flip notification. Therefore we can expect to hit
intel_finish_page_flip() under normal circumstances without a pending flip
and even without installing the pending_flip_obj. This is exacerbated by
aperture thrashing whilst binding the framebuffer

References:

  Bug 28079 - "glresize" causes kernel panic in intel_finish_page_flip.
  https://bugs.freedesktop.org/show_bug.cgi?id=28079

Reported-by: Nick Bowler <nbowler@draconx.ca>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: stable@kernel.org
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -4589,12 +4589,6 @@ void intel_finish_page_flip(struct drm_device *dev, int pipe)
 	spin_lock_irqsave(&dev->event_lock, flags);
 	work = intel_crtc->unpin_work;
 	if (work == NULL || !work->pending) {
-		if (work && !work->pending) {
-			obj_priv = to_intel_bo(work->pending_flip_obj);
-			DRM_DEBUG_DRIVER("flip finish: %p (%d) not pending?\n",
-					 obj_priv,
-					 atomic_read(&obj_priv->pending_flip));
-		}
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 		return;
 	}