drm/ttm: fix fence locking in ttm_buffer_object_transfer

Noticed while reviewing the fence locking in the radeon pageflip
handler.

v2: Instead of grabbing the bdev->fence_lock in object_transfer just
move the single callsite of that function a few lines, so that it is
protected by the fence_lock. Suggested by Jerome Glisse.

v3: Fix typo in commit message.
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -654,11 +654,13 @@ int ttm_bo_move_accel_cleanup(struct ttm_buffer_object *bo,
 		 */
 
 		set_bit(TTM_BO_PRIV_FLAG_MOVING, &bo->priv_flags);
+
+		/* ttm_buffer_object_transfer accesses bo->sync_obj */
+		ret = ttm_buffer_object_transfer(bo, &ghost_obj);
 		spin_unlock(&bdev->fence_lock);
 		if (tmp_obj)
 			driver->sync_obj_unref(&tmp_obj);
 
-		ret = ttm_buffer_object_transfer(bo, &ghost_obj);
 		if (ret)
 			return ret;