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>

drivers/gpu/drm/nouveau/nvkm/engine/device/user.c

@@ -346,6 +346,7 @@ nvkm_udevice_child_get(struct nvkm_object *object, int index,
 			return -EINVAL;
 
 		oclass->base = sclass->base;
+		oclass->engine = NULL;
 	}
 
 	oclass->ctor = nvkm_udevice_child_new;