drm/tests: helpers: Switch to EXPORT_SYMBOL_GPL

drm_kunit_device_init() among other things will allocate a device and
wrap around root_device_register. This function is exported with
EXPORT_SYMBOL_GPL, so we can't really change the license.

Fixes: a77a3ffa ("drm/tests: helpers: Add missing export")
Suggested-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-4-4615a663a84a@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>