drm/amd/display: create a function to decide mst link settings

[why]
create a dedicated function to make mst link settings decision, so that
the policy's decision is made in a unified place.

Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>