drm/fsl-dcu: handle missing panel gracefully

If the device tree property fsl,panel is missing, drm_panel_attach
is called with a NULL pointer as first argument. Having a panel is
basically mandatory since RGB is the only supported connector.
Check if a panel node has been found, return -ENODEV and cleanup
otherwise.
Signed-off-by: Stefan Agner <stefan@agner.ch>

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_rgb.c

@@ -171,14 +171,18 @@ int fsl_dcu_drm_connector_create(struct fsl_dcu_drm_device *fsl_dev,
 				      DRM_MODE_DPMS_OFF);
 
 	panel_node = of_parse_phandle(fsl_dev->np, "fsl,panel", 0);
-	if (panel_node) {
+	if (!panel_node) {
+		dev_err(fsl_dev->dev, "fsl,panel property not found\n");
+		ret = -ENODEV;
+		goto err_sysfs;
+	}
+
 	fsl_dev->connector.panel = of_drm_find_panel(panel_node);
 	if (!fsl_dev->connector.panel) {
 		ret = -EPROBE_DEFER;
-			goto err_sysfs;
+		goto err_panel;
 	}
 	of_node_put(panel_node);
-	}
 
 	ret = drm_panel_attach(fsl_dev->connector.panel, connector);
 	if (ret) {
@@ -188,6 +192,8 @@ int fsl_dcu_drm_connector_create(struct fsl_dcu_drm_device *fsl_dev,
 
 	return 0;
 
+err_panel:
+	of_node_put(panel_node);
 err_sysfs:
 	drm_connector_unregister(connector);
 err_cleanup: