drm/i915: enable atomic fb-helper

i915 supports enough atomic to have atomic fb-helper paths, even though
it does not yet advertise DRIVER_ATOMIC.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fbdev.c

@@ -689,6 +689,8 @@ int intel_fbdev_init(struct drm_device *dev)
 		return ret;
 	}
 
+	ifbdev->helper.atomic = true;
+
 	dev_priv->fbdev = ifbdev;
 	INIT_WORK(&dev_priv->fbdev_suspend_work, intel_fbdev_suspend_worker);