drm/vc4: Constify private state accessors

None of those helpers modify the pointed data, let's make them const.
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220328124304.2309418-4-maxime@cerno.tech

drivers/gpu/drm/vc4/vc4_kms.c

@@ -32,7 +32,8 @@ struct vc4_ctm_state {
 	int fifo;
 };
 
-static struct vc4_ctm_state *to_vc4_ctm_state(struct drm_private_state *priv)
+static struct vc4_ctm_state *
+to_vc4_ctm_state(const struct drm_private_state *priv)
 {
 	return container_of(priv, struct vc4_ctm_state, base);
 }
@@ -49,7 +50,7 @@ struct vc4_hvs_state {
 };
 
 static struct vc4_hvs_state *
-to_vc4_hvs_state(struct drm_private_state *priv)
+to_vc4_hvs_state(const struct drm_private_state *priv)
 {
 	return container_of(priv, struct vc4_hvs_state, base);
 }
@@ -61,7 +62,7 @@ struct vc4_load_tracker_state {
 };
 
 static struct vc4_load_tracker_state *
-to_vc4_load_tracker_state(struct drm_private_state *priv)
+to_vc4_load_tracker_state(const struct drm_private_state *priv)
 {
 	return container_of(priv, struct vc4_load_tracker_state, base);
 }