drm/nouveau/svm: initial support for shared virtual memory

This uses HMM to mirror a process' CPU page tables into a channel's page
tables, and keep them synchronised so that both the CPU and GPU are able
to access the same memory at the same virtual address.

While this code also supports Volta/Turing, it's only enabled for Pascal
GPUs currently due to channel recovery being unreliable right now on the
later GPUs.

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

drm/nouveau: remove left-over struct member

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

drm/nouveau/gem: attach fences to VMAs to track GPU usage

An upcoming patch will use these to fix issues related to the deferred
unmapping of GEM objects.

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

drm/nouveau/mmu: remove old vmm frontend

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

drm/nouveau: switch over to new memory and vmm interfaces

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

drm/nouveau: allocate vmm object for every client

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

drm/nouveau: separate constant-va tracking from nvkm vma structure

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