drm/amd/display: Use link_enc_cfg API for queries.

[Why]
The link_enc_cfg API operates in one of two modes depending on the stage
of application of dc_state to hardware. The API is the safest way to
query link encoder assignments.

[How]
Use results of link encoder assignment query using link_enc_cfg API.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>