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>

drivers/gpu/drm/i915/i915_drv.h

@@ -2339,6 +2339,7 @@ struct drm_i915_cmd_table {
 })
 #define INTEL_INFO(p) 	(&__I915__(p)->info)
 #define INTEL_DEVID(p)	(INTEL_INFO(p)->device_id)
+#define INTEL_REVID(p)	(__I915__(p)->dev->pdev->revision)
 
 #define IS_I830(dev)		(INTEL_DEVID(dev) == 0x3577)
 #define IS_845G(dev)		(INTEL_DEVID(dev) == 0x2562)
@@ -2387,6 +2388,11 @@ struct drm_i915_cmd_table {
 				 INTEL_DEVID(dev) == 0x0A1E)
 #define IS_PRELIMINARY_HW(intel_info) ((intel_info)->is_preliminary)
 
+#define SKL_REVID_A0		(0x0)
+#define SKL_REVID_B0		(0x1)
+#define SKL_REVID_C0		(0x2)
+#define SKL_REVID_D0		(0x3)
+
 /*
  * The genX designation typically refers to the render engine, so render
  * capability related checks should use IS_GEN, while display and other checks