drm/qxl: unset a pointer in sync_obj_unref

This fixes a BUG_ON(bo->sync_obj != NULL); in ttm_bo_release_list.

Cc: stable@vger.kernel.org #v3.10+
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/qxl/qxl_ttm.c

@@ -433,6 +433,7 @@ static int qxl_sync_obj_flush(void *sync_obj)
 
 static void qxl_sync_obj_unref(void **sync_obj)
 {
+	*sync_obj = NULL;
 }
 
 static void *qxl_sync_obj_ref(void *sync_obj)