In MDP5 Layer Mixer HW, the blender output is only the blended color
components (i.e R, G and B, or COLOR0/1/2 in MDP5 HW terminology). This
is fed to the BG input of the next blender. We also need to provide an
alpha (COLOR3) value for the BG input at the next stage.

This is configured via using the REG_MDP5_LM_BLEND_COLOR_OUT register.
For each stage, we can propagate either the BG or FG alpha to the next
stage.

The approach taken by the driver is to propagate FG alpha, if the plane
staged on that blender has an alpha. If it doesn't, we try to propagate
the base layer's alpha.

This is borrowed from downstream MDP5 kernel driver. Without this, we
don't see any cursor plane content.
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>