drm/nouveau/mmu: implement base for new vm management

This is the first chunk of the new VMM code that provides the structures
needed to describe a GPU virtual address-space layout, as well as common
interfaces to handle VMM creation, and connecting instances to a VMM.

The constructor now allocates the PD itself, rather than having the user
handle that manually.  This won't/can't be used until after all backends
have been ported to these interfaces, so a little bit of memory will be
wasted on Fermi and newer for a couple of commits in the series.

Compatibility has been hacked into the old code to allow each GPU backend
to be ported individually.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>