backend/drm: fix missing crtc->cursor NULL check

Fixes: cdb6fdbc6c ("backend/drm: remove missing cursor plane workaround")

backend/drm/drm.c

@@ -977,6 +977,9 @@ static bool drm_connector_move_cursor(struct wlr_output *output,
 		return false;
 	}
 	struct wlr_drm_plane *plane = conn->crtc->cursor;
+	if (!plane) {
+		return false;
+	}
 
 	struct wlr_box box = { .x = x, .y = y };