drm: Store the vblank timestamp when adjusting the counter during disable

During vblank disable the code tries to guess based on the
timestamps whether we just missed one vblank or not. And if so
it increments the counter. However it forgets to store the new
timestamp to the approriate slot in our timestamp ring buffer.
So anyone querying the timestamp for the resulting sequence
number would get a stale timestamp. Fix it up by storing the
new timestamp.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_irq.c

@@ -198,6 +198,13 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
 	 * hope for the best.
 	 */
 	if ((vblrc > 0) && (abs64(diff_ns) > 1000000)) {
+		/* Store new timestamp in ringbuffer. */
+		vblanktimestamp(dev, crtc, vblcount + 1) = tvblank;
+
+		/* Increment cooked vblank count. This also atomically commits
+		 * the timestamp computed above.
+		 */
+		smp_mb__before_atomic();
 		atomic_inc(&vblank->count);
 		smp_mb__after_atomic();
 	}