drm/mediatek: add mutex mod into ddp private data

drm/mediatek: add mutex mod into ddp private data

except mutex mod, mutex mod reg,mutex sof reg,
and mutex sof id will be ddp private data

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