drm/i915: ring w/a gen 9 revision definitions

Add Skylake stepping Revision IDs definitions.

v1: Use existing revision id.

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Use magic __I915__ and bikeshed #defines as suggested by
Damien.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>