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