drm/amd/display: add enable/disable dp link output to link_hwss

[why]
Factor enable/disable dp link output to link hwss.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>