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

When we added the support for the AUX channel in commit 13afcdd7
("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 6cba3fe4 ("drm/dp:
Add backpointer to drm_device in drm_dp_aux") but just for the
parade-ps8640.

Fixes: 13afcdd7

 ("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