drm/tests: helpers: Remove the name parameter

The device name isn't really useful, we can just define it instead of
exposing it in the API.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-6-4615a663a84a@cerno.tech


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