drm/exynos: dp: add of_graph dt binding support for panel

This patch adds of_graph dt binding support for panel device
and also keeps the backward compatibility.

i.e.,
The dts file for Exynos5800 based peach pi board
has a panel property so we need to keep the backward compatibility.

Changelog v3:
- bind only one of two nodes outbound - panel or bridge.

Changelog v2:
- return -EINVAL if getting a port node failed.
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>

drivers/gpu/drm/exynos/exynos_dp_core.c

@@ -1392,7 +1392,7 @@ static const struct component_ops exynos_dp_ops = {
 static int exynos_dp_probe(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
-	struct device_node *panel_node, *bridge_node, *endpoint;
+	struct device_node *panel_node = NULL, *bridge_node, *endpoint = NULL;
 	struct exynos_dp_device *dp;
 	int ret;
 
@@ -1403,13 +1403,31 @@ static int exynos_dp_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, dp);
 
+	/* This is for the backward compatibility. */
 	panel_node = of_parse_phandle(dev->of_node, "panel", 0);
 	if (panel_node) {
 		dp->panel = of_drm_find_panel(panel_node);
 		of_node_put(panel_node);
 		if (!dp->panel)
 			return -EPROBE_DEFER;
+	} else {
+		endpoint = of_graph_get_next_endpoint(dev->of_node, NULL);
+		if (endpoint) {
+			panel_node = of_graph_get_remote_port_parent(endpoint);
+			if (panel_node) {
+				dp->panel = of_drm_find_panel(panel_node);
+				of_node_put(panel_node);
+				if (!dp->panel)
+					return -EPROBE_DEFER;
+			} else {
+				DRM_ERROR("no port node for panel device.\n");
+				return -EINVAL;
 			}
+		}
+	}
+
+	if (endpoint)
+		goto out;
 
 	endpoint = of_graph_get_next_endpoint(dev->of_node, NULL);
 	if (endpoint) {
@@ -1423,6 +1441,7 @@ static int exynos_dp_probe(struct platform_device *pdev)
 			return -EPROBE_DEFER;
 	}
 
+out:
 	pm_runtime_enable(dev);
 
 	ret = component_add(&pdev->dev, &exynos_dp_ops);