drm/nouveau/mmu: support initialisation of client-managed address-spaces

NVKM is currently responsible for managing the allocation of a client's
GPU address-space, but there's various use-cases (ie. HMM address-space
mirroring) where giving a client more direct control is desirable.

This commit allows for a VMM to be created where the area allocated for
NVKM is limited to a client-specified window, the remainder of address-
space is controlled directly by the client.

Leaving a window is necessary to support various internal requirements,
but also to support existing allocation interfaces as not all of the HW
is capable of working with a HMM allocation.

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

drm/nouveau/mmu/gf100-: virtualise setting pdb base address for invalidation

It appears that Pascal and newer need something different.

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

drm/nouveau/mmu/gm200,gm20b: implement new vmm backend

Adds support for:
- 64KiB big page size.
- System-memory PTs.
- LPTE "invalid" state.
- (Tegra) Use of video memory aperture.
- Sparse PDEs/PTEs.
- Additional blocklinear kinds.

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

drm/nouveau/mmu/gm200,gm20b: implement vmm on top of new base

Adds support for:
- Per-VMM selection of big page size.
- System-memory PDs.

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