drm/tests: helpers: Make sure the device is bound

The device managed resources are freed when the device is detached, so
it has to be bound in the first place.

Let's create a fake driver that we will bind to our fake device to
benefit from the device managed cleanups in our tests.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-9-4615a663a84a@cerno.tech


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