drm/udl: Add ARGB8888 as a format

Even though the UDL driver converts to RGB565 internally (see
pixel32_to_be16() in udl_transfer.c), it advertises XRGB8888 for
compatibility. Let's add ARGB8888 to that list.

This makes UDL devices work on ChromeOS again after commit
c91acda3 ("drm/gem: Check for valid formats"). Prior to that
commit things were "working" because we'd silently treat the ARGB8888
that ChromeOS wanted as XRGB8888.

Fixes: c91acda3 ("drm/gem: Check for valid formats")
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227141928.1.I24ac8d51544e4624b7e9d438d95880c4283e611b@changeid