drm/vc4: Move HVS state to main header

In order to introduce unit tests for the HVS state computation, we'll
need access to the vc4_hvs_state struct definition and its associated
helpers.

Let's move them in our driver header.

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-v3-13-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>