drm/tests: helpers: Switch to a platform_device

The device managed resources are ran if the device has bus, which is not
the case of a root_device.

Let's use a platform_device instead.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-8-4615a663a84a@cerno.tech


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