drm/bridge/cdns-mhdp8546: Register DP aux channel with userspace

Just adds some missing calls to
drm_dp_aux_register()/drm_dp_aux_unregister() for when we attach/detach the
bridge.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423184309.207645-2-lyude@redhat.com


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