drm/amd/display: Secondary display goes blank on Non DCN31

[Why]
Due to integration issues with branch merging,
a regression happened that prevented secondary
displays from lighting up or enabling certain features

[How]
Separated the new logic to be for DCN31 only and retained
pre DCN31 logic for all other ASICs

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Ahmad Othman <Ahmad.Othman@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>