drm/i915/bxt: Add Broxton steppings

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_drv.h

@@ -2331,6 +2331,10 @@ struct drm_i915_cmd_table {
 #define SKL_REVID_D0		(0x3)
 #define SKL_REVID_E0		(0x4)
 
+#define BXT_REVID_A0		(0x0)
+#define BXT_REVID_B0		(0x3)
+#define BXT_REVID_C0		(0x6)
+
 /*
  * The genX designation typically refers to the render engine, so render
  * capability related checks should use IS_GEN, while display and other checks