drm/radeon/kms: use active device to pick connector for encoder

On the W500 we have UNIPHY routed to both DVI and DP, this seems
to always pick the DVI connector which means link training fails.

Switch to using active device to pick the connector, this seems
like it should be safe from a code review, and it fixes things
a bit more here.

Signed-off-by: Dave Airlie <airlied@redhat.com>