drm/nouveau: Fix ordering between TTM and GEM release

When the last reference to a TTM BO is dropped, ttm_bo_release() will
acquire the DMA reservation object's wound/wait mutex while trying to
clean up (ttm_bo_cleanup_refs_or_queue() via ttm_bo_release()). It is
therefore essential that drm_gem_object_release() be called after the
TTM BO has been uninitialized, otherwise drm_gem_object_release() has
already destroyed the wound/wait mutex (via dma_resv_fini()).

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