drm/tests: helpers: Allow for a custom device struct to be allocated

The current helper to allocate a DRM device doesn't allow for any
subclassing by drivers, which is going to be troublesome as we work on
getting some kunit testing on atomic modesetting code.

Let's use a similar pattern to the other allocation helpers by providing
the structure size and offset as arguments.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-10-4615a663a84a@cerno.tech


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