drm/mediatek: add ovl0/ovl_2l0 usecase

This patch add ovl0/ovl_2l0 usecase
in ovl->ovl_2l0 direct link usecase:
1. the crtc support layer number will 4+2
2. ovl_2l0 background color input select ovl0 when crtc init
and disable it when crtc finish
3. config ovl_2l0 layer, if crtc config layer number is
bigger than ovl0 support layers(max is 4)

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>