drm/tests: helpers: Rename the device init helper

The name doesn't really fit the conventions for the other helpers in
DRM/KMS, so let's rename it to make it obvious that we allocate a new
DRM device.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-5-4615a663a84a@cerno.tech


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