drm/radeon/kms: fix shared ddc handling

Connectors with a shared ddc line can be connected to different
encoders.

Reported by Pasi Kärkkäinen <pasik@iki.fi> on dri-devel

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