drm/amd/display: Save hpd_src for dc_link

Signed-off-by: Zeyu Fan <Zeyu.Fan@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>