drm/mediatek: gamma: Add kerneldoc for struct mtk_disp_gamma

The mtk_disp_gamma structure was completely undocumented: add some
kerneldoc documentation to it.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231012095736.100784-16-angelogioacchino.delregno@collabora.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>