nouveau: move more abi16-specific logic into abi16.c

v2.
- add a comment about the (ab)use of nouveau_object::length
- add a comment about abi16_object() return values
v3.
- handle new client + old kernel for sw classes

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>