drm/medaitek: add layer_nr for ovl private data

This patch add layer_nr for ovl private data
ovl_2l almost same with with ovl hardware, except the
layer number for ovl_2l is 2 and ovl is 4.
this patch is a preparation for ovl-2l and
ovl share the same driver.

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