dt-bindings: display: mediatek: revise enum to const

There won't be more than 1 fallback for these bindings, so we modify
them to use const instead of enum.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220406030009.2357-2-rex-bc.chen@mediatek.com/


Signed-off-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>