drm/nouveau/fifo/gk104: fix race condition when updating engine runlists

The CPU-side tracking of engine runlists was not protected by a lock,
leading to list corruption, eventually causing runlist_update() to
overrun the GPU-side runlist, triggering an OOPS.

Fixes some of the issues noticed during parallel piglit runs.

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