backend/drm: ensure plane surfaces are cleaned up on shutdown

Right now this is done "by chance" because we disable all CRTCs
on shutdown. However, we'll stop doing this. Plus, if disabling
a CRTC fails, we don't cleanup properly.

backend/drm/drm.c

@@ -328,6 +328,7 @@ void finish_drm_resources(struct wlr_drm_backend *drm) {
 
 	for (size_t i = 0; i < drm->num_planes; ++i) {
 		struct wlr_drm_plane *plane = &drm->planes[i];
+		drm_plane_finish_surface(plane);
 		wlr_drm_format_set_finish(&plane->formats);
 	}