drm/mediatek: add mutex mod register offset into ddp private data

drm/mediatek: add mutex mod register offset into ddp private data

mutex mod register offset will be private data of ddp.

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