drm/nouveau: detect disabled device in irq handler and return IRQ_NONE

Signed-off-by: Ben Skeggs <[e-mail]>

drivers/gpu/drm/nouveau/nouveau_irq.c

···
 	int i;
 
 	stat = nv_rd32(dev, NV03_PMC_INTR_0);
-	if (!stat)
+	if (stat == 0 || stat == ~0)
 		return IRQ_NONE;
 
 	spin_lock_irqsave(&dev_priv->context_switch_lock, flags);