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>

diff --cc drivers/gpu/drm/radeon/radeon_encoders.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_mode.h
Simple merge