drm/atomic: Constify the old/new state accessors

The drm_atomic_get_(old|new)_*_state don't modify the passed
drm_atomic_state, so we can make it 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-v1-9-051a0bb60a16@cerno.tech