drm/nouveau: allocate kernel's notifier object at end of block

The nv30/nv40 3d driver is about to start using DMA_FENCE from the 3D
object which, it turns out, doesn't like its DMA object to not be
aligned to a 4KiB boundary.

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