drm/bridge/analogix/anx78xx: Add missing drm_dp_aux_unregister() call

Surprisingly, this bridge actually registers it's AUX adapter at the
correct time already. Nice job! However, it does forget to actually
unregister the AUX adapter, so let's add a bridge function to handle that.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219215326.2227596-8-lyude@redhat.com