drm/i915: Fix bsd2 ring name

Chris Wilson noticed the "bds2" typo.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1452619956-27014-3-git-send-email-tvrtko.ursulin@linux.intel.com