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>