drm/atomic: Update old_fb after setting a property.

This change updates the old_fb pointer only after acquiring the plane lock,
if there are no properties the fb cannot have been changed either, so
this works out correctly.

Found in a discussion with Rob Clark.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_atomic.c

@@ -1472,12 +1472,6 @@ retry:
 			goto fail;
 		}
 
-		if (obj->type == DRM_MODE_OBJECT_PLANE) {
-			plane = obj_to_plane(obj);
-			plane_mask |= (1 << drm_plane_index(plane));
-			plane->old_fb = plane->fb;
-		}
-
 		if (get_user(count_props, count_props_ptr + copied_objs)) {
 			ret = -EFAULT;
 			goto fail;
@@ -1514,6 +1508,12 @@ retry:
 
 			copied_props++;
 		}
+
+		if (obj->type == DRM_MODE_OBJECT_PLANE && count_props) {
+			plane = obj_to_plane(obj);
+			plane_mask |= (1 << drm_plane_index(plane));
+			plane->old_fb = plane->fb;
+		}
 	}
 
 	if (arg->flags & DRM_MODE_PAGE_FLIP_EVENT) {