drm/amd/display: add HDR visual confirm

[Why]
Testing team wants a way to tell if HDR is on or not

[How]
Program the overscan color to visually indicate the HDR state of the top-most plane

Signed-off-by: Gloria Li <geling.li@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>