dt-bindings: display: mediatek, hdmi: Convert to use graph schema

Update the mediatek,dpi binding to use the graph schema.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>