drm/i915: Grab dev->struct_mutex in i915_gem_pageflip_info

We could walk of a bad list otherwise when someone concurrently
unbinds stuff for fun.

I've suspected this as the root-cause behind seemingly inconsistent
state, but alas it's not.
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -513,6 +513,11 @@ static int i915_gem_pageflip_info(struct seq_file *m, void *data)
 	struct drm_device *dev = node->minor->dev;
 	unsigned long flags;
 	struct intel_crtc *crtc;
+	int ret;
+
+	ret = mutex_lock_interruptible(&dev->struct_mutex);
+	if (ret)
+		return ret;
 
 	for_each_intel_crtc(dev, crtc) {
 		const char pipe = pipe_name(crtc->pipe);
@@ -554,6 +559,8 @@ static int i915_gem_pageflip_info(struct seq_file *m, void *data)
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 	}
 
+	mutex_unlock(&dev->struct_mutex);
+
 	return 0;
 }