drm/nouveau: use vmalloc for pgt allocation

commit d005f51eb93d71cd40ebd11dd377453fa8c8a42a upstream.

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>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>