drm/tests: helpers: Allow to pass a custom drm_driver

Some tests will need to provide their own drm_driver instead of relying
on the dumb one in the helpers, so let's create a helper that allows to
do so.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-11-4615a663a84a@cerno.tech


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