drm/nouveau/fb/ram: add interface to allocate vram as an nvkm_memory object

Upcoming MMU changes use nvkm_memory as its basic representation of memory,
so we need to be able to allocate VRAM like this.

The code is basically identical to the current chipset-specific allocators,
minus support for compression tags (which will be handled elsewhere anyway).

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