backend/drm: downgrade test-only commit failure log level

Let's not clutter the logs with error messages when a test-only
atomic commit fails.

backend/drm/atomic.c

@@ -31,8 +31,10 @@ static bool atomic_commit(struct atomic *atom,
 	}
 
 	int ret = drmModeAtomicCommit(drm->fd, atom->req, flags, drm);
-	if (ret) {
-		wlr_drm_conn_log_errno(conn, WLR_ERROR, "Atomic %s failed (%s)",
+	if (ret != 0) {
+		wlr_drm_conn_log_errno(conn,
+			(flags & DRM_MODE_ATOMIC_TEST_ONLY) ? WLR_DEBUG : WLR_ERROR,
+			"Atomic %s failed (%s)",
 			(flags & DRM_MODE_ATOMIC_TEST_ONLY) ? "test" : "commit",
 			(flags & DRM_MODE_ATOMIC_ALLOW_MODESET) ? "modeset" : "pageflip");
 		return false;