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>

drivers/gpu/drm/nouveau/nouveau_irq.c

@@ -41,12 +41,8 @@
 void
 nouveau_irq_preinstall(struct drm_device *dev)
 {
-	struct drm_nouveau_private *dev_priv = dev->dev_private;
-
 	/* Master disable */
 	nv_wr32(dev, NV03_PMC_INTR_EN_0, 0);
-
-	INIT_LIST_HEAD(&dev_priv->vbl_waiting);
 }
 
 int