drm/bridge/ti-sn65dsi86: (Un)register aux device on bridge attach/detach

Since we're about to add a back-pointer to drm_dev in drm_dp_aux, let's
move the AUX adapter registration to the first point where we know which
DRM device we'll be working with - when the drm_bridge is attached.
Likewise, we unregister the AUX adapter on bridge detachment by adding a
ti_sn_bridge_detach() callback.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210219215326.2227596-7-lyude@redhat.com