drm/msm: mdp4 lvds: get panel node via of graph parsing

We currently get the output connected to LVDS by looking for a phandle
called 'qcom,lvds-panel' under the mdp DT node.

Use the more standard of_graph approach to create an lvds output port,
and retrieve the panel node from the port's endpoint data.

v3
- Fix return value checks of of_graph_* calls.

Tested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>
diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
index 12c97cf..3be7a56 100644
--- a/drivers/gpu/drm/msm/msm_drv.h
+++ b/drivers/gpu/drm/msm/msm_drv.h
@@ -30,6 +30,7 @@
 #include <linux/list.h>
 #include <linux/iommu.h>
 #include <linux/types.h>
+#include <linux/of_graph.h>
 #include <asm/sizes.h>
 
 #ifndef CONFIG_OF