drm/radeon/kms/atom: add proper external encoders support
These are external encoder chips connected via DVO or DP.
The actual external encoder programming is handled by the
kms encoder functions for primary encoder.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
authored by Alex Deucher and committed by Dave Airlie 3e4b9982 99999aaa