drm/bridge/analogix/anx6345: Add missing drm_dp_aux_unregister() call

Another driver I found that seems to forget to unregister it's DP AUX
device. Let's fix this by adding anx6345_bridge_detach().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219215326.2227596-11-lyude@redhat.com