drm/nouveau: init vblank requests list

Fixes kernel panic when vblank interrupt triggers before first sync to
vblank request.

(Besides init, remove some relevant leftovers from vblank rework)

Reported-by: Ortwin Glück <odi@odi.ch>
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: stable@vger.kernel.org [3.5]
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>