drm/nouveau: disable copy engine on NVAF

The copy engine exhibits random memory corruption in at least one case, the
GeForce 320M (nv50, 0xaf) in the MacBookAir3,1.

This patch omits creating the engine for the specific chipset, falling back
to M2MF, which kills the symptoms.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>