drm/nouveau: allocate vram with nvkm_ram_get()

This will cause a subtle behaviour change on GPUs that are in mixed-memory
configurations in that VRAM in the degraded section of VRAM will no longer
be used for TTM buffer objects.

That section of VRAM is not meant to be used for displayable/compressed
surfaces, and we have no reliable way with the current interfaces to be
able to make that decision properly.

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