drm/amd/display: Use correct DTO_SRC_SEL for 128b/132b encoding

[WHY]
DP DTO isn't used for 128b/132b encoding

[HOW]
Check current link rate to determine whether using 8b/10b or 128/132b encoding

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>