drm: bridge: analogix: Unregister dp aux when unbinding

The dp aux is registered when binding analogix dp.

Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491481885-13775-3-git-send-email-jeffy.chen@rock-chips.com