drm/i915: initialize the parity work only once

This fixes an (albeit really hard to hit) race resulting in an oops:
- The parity work get scheduled.
- We re-init the irq state and call INIT_WORK again.
- The workqueue code tries to run the work item and stumbles over a
  work item that should be on it's runlist.

Also initiliaze the work item unconditionally like all the others,
it's simpler.
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -1723,10 +1723,6 @@ static void ironlake_irq_preinstall(struct drm_device *dev)
 
 	atomic_set(&dev_priv->irq_received, 0);
 
-
-	if (IS_IVYBRIDGE(dev))
-		INIT_WORK(&dev_priv->parity_error_work, ivybridge_parity_work);
-
 	I915_WRITE(HWSTAM, 0xeffe);
 
 	/* XXX hotplug from PCH */
@@ -2647,6 +2643,7 @@ void intel_irq_init(struct drm_device *dev)
 	INIT_WORK(&dev_priv->hotplug_work, i915_hotplug_work_func);
 	INIT_WORK(&dev_priv->error_work, i915_error_work_func);
 	INIT_WORK(&dev_priv->rps_work, gen6_pm_rps_work);
+	INIT_WORK(&dev_priv->parity_error_work, ivybridge_parity_work);
 
 	dev->driver->get_vblank_counter = i915_get_vblank_counter;
 	dev->max_vblank_count = 0xffffff; /* only 24 bits of frame count */