drm/nouveau: fix allocation of notifier object

Commit 73412c38

 ("drm/nouveau: allocate
kernel's notifier object at end of block") intended to align end of
notifier block to page boundary, but start of block was miscalculated
to be off by -16 bytes. Fix it.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>