dt-bindings: display: mediatek: gamma: Change MT8195 to single enum group

Since MT8195 gamma has multiple bank for 12 bits LUT and it is
different from any other SoC LUT setting.

So we add MT8195 compatible to the single enum group to make its
driver data settings can be reused by other SoC.

Signed-off-by: Jason-JH.Lin <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: AngeloGioacchino Del Regno <[e-mail]>
Reviewed-by: CK Hu <[e-mail]>
Link: https://patchwork.kernel.org/project/dri-devel/patch/[e-mail]/
Signed-off-by: Chun-Kuang Hu <[e-mail]>

Documentation/devicetree/bindings/display/mediatek/mediatek,gamma.yaml

···
       - enum:
           - mediatek,mt8173-disp-gamma
           - mediatek,mt8183-disp-gamma
+          - mediatek,mt8195-disp-gamma
       - items:
           - enum:
               - mediatek,mt6795-disp-gamma