drm/vblank: Use abs64(diff_ns) for s64 diff_ns instead of abs(diff_ns)

Use of abs() wrongly wrapped diff_ns to 32 bit, which gives a 1/4000
probability of a missed vblank increment at each vblank irq reenable
if the kms driver doesn't support high precision vblank timestamping.
Not a big deal in practice, but let's make it nice.
Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -164,7 +164,7 @@ static void vblank_disable_and_save(struct drm_device *dev, int crtc)
 	 * available. In that case we can't account for this and just
 	 * hope for the best.
 	 */
-	if ((vblrc > 0) && (abs(diff_ns) > 1000000))
+	if ((vblrc > 0) && (abs64(diff_ns) > 1000000))
 		atomic_inc(&dev->_vblank_count[crtc]);
 
 	/* Invalidate all timestamps while vblank irq's are off. */
@@ -1293,7 +1293,7 @@ bool drm_handle_vblank(struct drm_device *dev, int crtc)
 	 * e.g., due to spurious vblank interrupts. We need to
 	 * ignore those for accounting.
 	 */
-	if (abs(diff_ns) > DRM_REDUNDANT_VBLIRQ_THRESH_NS) {
+	if (abs64(diff_ns) > DRM_REDUNDANT_VBLIRQ_THRESH_NS) {
 		/* Store new timestamp in ringbuffer. */
 		vblanktimestamp(dev, crtc, vblcount + 1) = tvblank;
 		smp_wmb();