drm/bridge/analogix/dp_core: Unregister DP AUX channel on error in analogix_dp_probe()

Just another drive-by fix I noticed while going through the tree to cleanup
DP aux adapter registration - make sure we unregister the DP AUX dev if
analogix_dp_probe() fails.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219215326.2227596-14-lyude@redhat.com