drm/nouveau: add nvif_mmu to nouveau_drm

This allocates a new nvif_mmu in nouveau_drm, and uses it for TTM
backend memory allocations instead of nouveau_drm.master.mmu,
which is removed by a later commit that removes nouveau_drm.master
entirely.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-29-bskeggs@nvidia.com