drm/nouveau: use vmalloc for pgt allocation

Page tables on nv50 take 48kB, which can be hard to allocate in one piece.
Let's use vmalloc.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@vger.kernel.org [3.7+]
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>