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

Update the mediatek,dpi binding to use the graph schema. Missed
this one from the mass conversion since it's not part of drm-misc.

Cc: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Jitao shi <jitao.shi@mediatek.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mediatek@lists.infradead.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>