drm/nouveau/mmu: remove support for old backends

drivers/gpu/drm/nouveau/include/nvkm/subdev/fb.h

drivers/gpu/drm/nouveau/include/nvkm/subdev/mmu.h

drivers/gpu/drm/nouveau/nouveau_mem.c

drivers/gpu/drm/nouveau/nouveau_mem.h

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/base.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/g84.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/gf100.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/gk104.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/gk20a.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/gm200.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/gm20b.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/nv04.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/nv41.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/nv44.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/nv50.c

drivers/gpu/drm/nouveau/nvkm/subdev/mmu/priv.h