drm/nouveau: Don't take dev->struct_mutex in ttm_fini

This is only called in driver load/unload paths, no need to grab any
locks at all. Also, ttm takes care of itself anyway.

Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>