drm/i915: fix intel_fb_obj_flush documentation

Reported by the kbuild test robot.

Regression introduced by:
commit de152b62
Author: Rodrigo Vivi <rodrigo.vivi@intel.com>
Date:   Tue Jul 7 16:28:51 2015 -0700
    drm/i915: Add origin to frontbuffer tracking flush

(I reviewed this commit, so it's also my fault)
Signed-off-by: NPaulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: NRodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_frontbuffer.c

@@ -136,6 +136,7 @@ void intel_frontbuffer_flush(struct drm_device *dev,
  * intel_fb_obj_flush - flush frontbuffer object
  * @obj: GEM object to flush
  * @retire: set when retiring asynchronous rendering
+ * @origin: which operation caused the flush
  *
  * This function gets called every time rendering on the given object has
  * completed and frontbuffer caching can be started again. If @retire is true