drm/nouveau/nvkm: zero out engine pointer for subdev-provided classes

Doesn't fix any known issue, but noticed fifo being initialised in
logs in response to mmu allocation.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>