drm/amd/display: Update background color in bottommost mpcc

[Why]
Background color only takes effect in bottommost mpcc.

[How]
Update background color in bottommost mpcc.

Signed-off-by: Hugo Hu <hugo.hu@amd.com>
Reviewed-by: Yongqiang Sun <yongqiang.sun@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>