drm/amd/display: use encoder type independent hwss instead of accessing enc directly

[why]
in dc_link_dp there still exist a few places where we call dio encoders
without checking current enabled encoder type.
The change is to make these places to call hwss equivalent functions so
it won't mistakenly program a wrong type encoder.

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>