drm/exynos: hdmi: consider legacy dt binding

This patch considers legacy dt binding, and resolves
the issue that the use of existing dtb is broken.

To resove the dt broken issue, this path tries to get
legacy dt nodes from existing dtb directly prior to
getting new dt nodes.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>