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: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>