drm/amd/display: rename dc_link_hwss struct to link_hwss

[why]
This is internal to dc so we don't have to add a prefix to indicate it
is dc link_hwss instead someone else link_hwss.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>