drm/i915: Remove extra "ring"

Sadly, this isn't the first time we've done this:
http://lists.freedesktop.org/archives/intel-gfx/2013-June/029065.html

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>