drm/vc4: kms: Constify the HVS old/new state helpers

commit  e9ebbd3472482d4c8ec5717b78946025f3976dc7 from
https://github.com/raspberrypi/linux.git rpi-6.1.y

The vc4_hvs_get_(old|new)_global_state functions don't modify the
drm_atomic_state passed as an argument, so let's make it const.

Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v1-13-051a0bb60a16@cerno.tech
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Meng Li <Meng.Li@windriver.com>