drm/nouveau/mmu: implement new vmm frontend

drm/nouveau/mmu: implement new vmm frontend

These are the new priviledged interfaces to the VMM backends, and expose
some functionality that wasn't previously available.

It's now possible to allocate a chunk of address-space (even all of it),
without causing page tables to be allocated up-front, and then map into
it at arbitrary locations.  This is the basic primitive used to support
features such as sparse mapping, or to allow userspace control over its
own address-space, or HMM (where the GPU driver isn't in control of the
address-space layout).

Rather than being tied to a subtle combination of memory object and VMA
properties, arguments that control map flags (ro, kind, etc) are passed
explicitly at map time.

The compatibility hacks to implement the old frontend on top of the new
driver backends have been replaced with something similar to implement
the old frontend's interfaces on top of the new frontend.

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