drm/mediatek: add function to return OVL layer number

This patch add function to return OVL layer number

For now, MT8173, MT2712, MT2701 OVL all has 4 layer.

Signed-off-by: Stu Hsieh <stu.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>