drm/radeon/kms/atom/dce3: call transmitter init on mode set

Generally this is done at post, but might not always be done
with softboot or for connectors on docking stations.

Could probably be done once when the driver loads/resumes
rather than on each mode set.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>