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>