drm/nv44/mpeg: create a copy of the nv31/nv40 impls

The nv31/nv40 impls are actually fairly nv44-specific, since they assume
the presence of the instance register/context switching. Create a copy
before nv31/nv40 get fixed.

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