drm/bridge/analogix/anx78xx: Setup encoder before registering connector

Since encoder mappings for connectors are exposed to userspace, we should
be attaching the encoder before exposing the connector to userspace. Just a
drive-by fix for an issue I noticed while fixing up usages of
drm_dp_aux_init()/drm_dp_aux_register() across the tree.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219215326.2227596-9-lyude@redhat.com