drm/amd/display: move get_link_hwss to dc_resource

[why]
Isolate the way to obtain link_hwss from the actual implemenation of
link_hwss. So the caller can call link_hwss without knowing the
implementation detail of link_hwss.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Stylon Wang <stylon.wang@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>