drm/amd/display: Add override for visual confirm

drm/amd/display: Add override for visual confirm

[WHY]
We want to allow the display manager to override the visual
confirm color in DC when required.

[HOW]
Add new visual confirm mode VISUAL_CONFIRM_EXPLICIT, check mode before
setting visual confirm color.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Leo Zeng <Leo.Zeng@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>