drm/nouveau/core: use vzalloc for allocating ramht

Most calls to nvkm_ramht_new use 0x8000 as the size. This results in a
fairly sizeable chunk of memory to be allocated, which may not be
available with kzalloc. Since this is done fairly rarely (once per
channel), use vzalloc instead.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>