drm: Don't clear vblank timestamps when vblank interrupt is disabled

Clearing the timestamps causes us to send zeroed timestamps to userspace
if they get sent out in response to the drm_vblank_off(). It's better
to send the very latest timestamp and count instead.

Testcase: igt/kms_flip/modeset-vs-vblank-race
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_irq.c

@@ -55,14 +55,6 @@
  */
 #define DRM_REDUNDANT_VBLIRQ_THRESH_NS 1000000
 
-/*
- * Clear vblank timestamp buffer for a crtc.
- */
-static void clear_vblank_timestamps(struct drm_device *dev, int crtc)
-{
-	memset(dev->vblank[crtc].time, 0, sizeof(dev->vblank[crtc].time));
-}
-
 /*
  * Disable vblank irq's on crtc, make sure that last vblank count
  * of hardware and corresponding consistent software vblank counter
@@ -131,9 +123,6 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
 		smp_mb__after_atomic();
 	}
 
-	/* Invalidate all timestamps while vblank irq's are off. */
-	clear_vblank_timestamps(dev, crtc);
-
 	spin_unlock_irqrestore(&dev->vblank_time_lock, irqflags);
 }