drm/mediatek: add gmc_bits for ovl private data

This patch add gmc_bits for ovl private data
GMC register was set RDMA ultra and pre-ultra threshold.
10bit GMC register define is different with other SOC, gmc_thrshd_l not
used.

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