drm/mediatek: aal: Add kerneldoc for struct mtk_disp_aal

The kerneldoc for struct mtk_disp_aal is missing: write one and
document this structure.

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/20231012095736.100784-15-angelogioacchino.delregno@collabora.com/


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