dt-bindings: display: mediatek: change to use enum for mutex

All single entry cases in mutex can be merged as a single enum.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220406030009.2357-3-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>