drm/amd/display: Refactor visual confirm

[Why + How]
Visual confirm has no asic-specific logic,
so we can refactor and unify these functions
that are currently spread out across multiple
dcn files.
Add a new hw sequencer interface update_visual_confirm_color,
and a new mpc function pointer set_bg_color.
This will allow visual confirm to updated independently
of MPCC blending updates.

v2: squash in DCN3.1 fixes

Signed-off-by: Wyatt Wood <wyatt.wood@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>