drm/bridge: ti-sn65dsi86: use the auxiliary device

The auxiliary device creation of this driver is simple enough to
use the available auxiliary device creation helper.

Use it and remove some boilerplate code.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20250218-aux-device-create-helper-v4-3-c3d7dfdea2e6@baylibre.com