drm/i915: add a new BSD ring buffer for Sandybridge

This ring buffer is used for video decoding/encoding on Sandybridge.

Signed-off-by: Xiang, Haihao <haihao.xiang@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>