drm/nouveau/core/memory: split info pointers from accessor pointers

drm/nouveau/core/memory: split info pointers from accessor pointers

The accessor functions can change as a result of acquire()/release() calls,
and are protected by any refcounting done there.

Other functions must remain constant, as they can be called any time.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>