This patch add background color input select function for ovl/ovl_2l

ovl include 4 DRAM layer and 1 background color layer
ovl_2l include 4 DRAM layer and 1 background color layer
DRAM layer frame buffer data from render hardware, GPU for example.
backgournd color layer is embed in ovl/ovl_2l, we can only set
it color, but not support DRAM frame buffer.

for ovl0->ovl0_2l direct link usecase,
we need set ovl0_2l background color intput select from ovl0
if render send DRAM buffer layer number <=4, all these layer read
by ovl.
layer0 is at the bottom of all layers.
layer3 is at the top of all layers.
if render send DRAM buffer layer numbfer >=4 && <=6
ovl0 read layer0~3
ovl0_2l read layer4~5
layer5 is at the top ot all these layers.

the decision of how to setting ovl0/ovl0_2l read these layer data
is controlled in mtk crtc, which will be another patch
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>