drm/radeon/kms: use hardcoded dig encoder to transmitter mapping for DCE4.1

The encoders are supposedly fully routeable, but changing the mapping
doesn't always seem to take.  Using a hardcoded mapping is much more
reliable.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=41366

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>