backend/drm: use atomic test-only commits for direct scan-out

This allows callers to use wlr_output_test to figure out whether a
buffer can be scanned out prior to committing the output.

backend/drm/drm.c

@@ -399,13 +399,11 @@ static bool test_buffer(struct wlr_drm_connector *conn,
 		return false;
 	}
 
-	struct wlr_drm_fb *fb = NULL;
+	if (!drm_fb_import(&crtc->primary->pending_fb, drm, wlr_buffer,
-	if (!drm_fb_import(&fb, drm, wlr_buffer, &crtc->primary->formats)) {
+			&crtc->primary->formats)) {
 		return false;
 	}
-	drm_fb_clear(&fb);
+	return drm_crtc_commit(conn, DRM_MODE_ATOMIC_TEST_ONLY);
-
-	return true;
 }
 
 static bool drm_connector_test(struct wlr_output *output) {