drm/nouveau: Do not leak client objects

The memory allocated for a nouveau_cli object in nouveau_cli_create() is
never freed. Free the memory in nouveau_cli_destroy() to plug this leak.

kmemleak recorded this after running a couple of nouveau test programs.
Note that kmemleak points at drm_open_helper() because for some reason
it thinks that skipping the first two stack frames is a good idea.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>