drm/bridge: ti-sn65dsi86: Add support for the DP AUX bus

We want to provide our panel with access to the DP AUX channel. The
way to do this is to let our panel be a child of ours using the fancy
new DP AUX bus support.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.8.Ib5fe0638da85800141ce141bb8e441c5f25438d4@changeid