drm/vc4: hvs: Provide a function to initialize the HVS structure

We'll need to initialize the HVS structure without a backing device to
create a mock we'll use for testing.

Split the structure initialization part into a separate function.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-16-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>