drm/nv40/mpeg: write magic value to channel object to make it work

Looks like the rewrite in commit ebb945a9 ("drm/nouveau: port all
engines to new engine module format") missed that one little detail.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>