drm/mediatek: add function to background color input select for ovl/ovl_2l direct link

This patch add function to background color input select for ovl/ovl_2l
direct link for ovl/ovl_2l direct link usecase, we need set background
color input select for these hardware. This is preparation patch for
ovl/ovl_2l usecase.

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