drm/nouveau/instmem/gk20a: fix race conditions

The LRU list used for recycling CPU mappings was handling concurrency
very poorly. For instance, if an instobj was acquired twice before being
released once, it would end up into the LRU list even though there is
still a client accessing it.

This patch fixes this by properly counting how many clients are
currently using a given instobj.

While at it, we also raise errors when inconsistencies are detected, and
factorize some code.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>