drm/radeon/kms/DCE4.1: fix dig encoder to transmitter mapping

llano has fully routeable dig encoders similar to DCE3.2 while
ontario has a hardcoded mapping similar to DCE4.0.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>