drm/i915: fix vblank wait test condition

When converting this to the new wait_for macro I inverted the wait
condition, which causes all sorts of problems.  So correct it to fix
several failures caused by the bad wait (flickering, bad output
detection, tearing, etc.).
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Tested-by: NSitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/intel_display.c

@@ -992,7 +992,7 @@ void intel_wait_for_vblank(struct drm_device *dev, int pipe)
 
 	/* Wait for vblank interrupt bit to set */
 	if (wait_for((I915_READ(pipestat_reg) &
-		      PIPE_VBLANK_INTERRUPT_STATUS) == 0,
+		      PIPE_VBLANK_INTERRUPT_STATUS),
 		     50, 0))
 		DRM_DEBUG_KMS("vblank wait timed out\n");
 }