drm/amdgpu/display: don't call drm_dp_mst_connector_late_register (v2)

Nothing else calls it.  Not sure it's necessary.

v2: remove unused port variable.

Reviewed-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>