drm/amd/display: temporarly move non link_hwss code to dc_link_dp

[why]
Clean up dc_link_hwss file in the preparation of breaking it down to
file for each encoder type. We temporarly move the original dp link
functions in link_hwss back to dc_link_dp. We will break dc_link_dp down
after link_hwss is in good shape.

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>