drm/tests: helpers: Document drm_kunit_device_init()

Commit 44a39283 ("drm/tests: Add Kunit Helpers") introduced the
drm_kunit_device_init() function but didn't document it properly. Add
that documentation.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>