drm/amd/display: Add support for visual confirm color

[Why]
We want to get the visual confirm color of the bottom-most pipe
for test automation.

[How]
Save the visual confirm color to plane_state before program to MPC;

Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Brian Chang <Brian.Chang@amd.com>
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>