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

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.techReviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/vc4/vc4_kms.c

@@ -191,7 +191,7 @@ vc4_ctm_commit(struct vc4_dev *vc4, struct drm_atomic_state *state)
 }
 
 static struct vc4_hvs_state *
-vc4_hvs_get_new_global_state(struct drm_atomic_state *state)
+vc4_hvs_get_new_global_state(const struct drm_atomic_state *state)
 {
 	struct vc4_dev *vc4 = to_vc4_dev(state->dev);
 	struct drm_private_state *priv_state;
@@ -204,7 +204,7 @@ vc4_hvs_get_new_global_state(struct drm_atomic_state *state)
 }
 
 static struct vc4_hvs_state *
-vc4_hvs_get_old_global_state(struct drm_atomic_state *state)
+vc4_hvs_get_old_global_state(const struct drm_atomic_state *state)
 {
 	struct vc4_dev *vc4 = to_vc4_dev(state->dev);
 	struct drm_private_state *priv_state;