drm: Delete the vblank timer synchronously at cleanup time

A race condition exists in drm_vblank_cleanup() if the vblank disable
timer callback runs after freeing the memory that its callback function
tries to access. Fix this by deleting the timer synchronously.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -189,7 +189,7 @@ void drm_vblank_cleanup(struct drm_device *dev)
 	if (dev->num_crtcs == 0)
 		return;
 
-	del_timer(&dev->vblank_disable_timer);
+	del_timer_sync(&dev->vblank_disable_timer);
 
 	vblank_disable_fn((unsigned long)dev);