drm/amd/display: expose dsc overhead bw in dc dsc header

[why]
DM needs to know how much overhead is added to DSC as result
of AMD internal DSC limitation.

Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: George Shen <george.shen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>