drm/amd/display: Avoid create MST prop after registration

[Why]
Prop are created at boot stage, and not allowed to create new prop
after device registration.

[How]
Reuse the connector property from SST if exist.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>