drm/bridge: parade-ps8640: Add backpointer to drm_device in drm_dp_aux

drm/bridge: parade-ps8640: Add backpointer to drm_device in drm_dp_aux

When we added the support for the AUX channel in commit 042e1311f2d9
("drm/bridge: parade-ps8640: Add support for AUX channel") we forgot
to set "drm_dev" to avoid the warning splat at the beginning of
drm_dp_aux_register(). Since everything was working I guess I never
noticed the splat when testing against mainline. In any case, it's
easy to fix. This is basically just like commit f915ed7eeb3a ("drm/dp:
Add backpointer to drm_device in drm_dp_aux") but just for the
parade-ps8640.

Fixes: 042e1311f2d9 ("drm/bridge: parade-ps8640: Add support for AUX channel")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Philip Chen <philipchen@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206162907.1.I1f5d1eba741e4663050ec1b8e39a753f6e42e38b@changeid