v2: Use for_each_ring() (Chris)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446725633-6419-1-git-send-email-ville.syrjala@linux.intel.comReviewed-by: NChris Wilson <chris@chris-wilson.co.uk>