drm/vc4: Constify container_of wrappers

None of our wrappers around container_of to access our objects from the
DRM object pointer actually modify the latter.

Let's make them const.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123-rpi-kunit-tests-v1-11-051a0bb60a16@cerno.tech