i915: initialize the free_list of the fencing atomic_helper

Just in case the llist model changes and NULL isn't valid
initialization.
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170406232347.988-4-aarcange@redhat.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -14992,6 +14992,7 @@ int intel_modeset_init(struct drm_device *dev)
 
 	dev->mode_config.funcs = &intel_mode_funcs;
 
+	init_llist_head(&dev_priv->atomic_helper.free_list);
 	INIT_WORK(&dev_priv->atomic_helper.free_work,
 		  intel_atomic_helper_free_state_worker);