drm/i915: Add early BXT sdv to the list of preproduction machines

Extend intel_detect_preproduction_hw() to include BXT A and B steppings.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: "Zanoni, Paulo R" <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170130104458.2653-3-chris@chris-wilson.co.uk
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_drv.c

@@ -783,6 +783,7 @@ static void intel_detect_preproduction_hw(struct drm_i915_private *dev_priv)
 
 	pre |= IS_HSW_EARLY_SDV(dev_priv);
 	pre |= IS_SKL_REVID(dev_priv, 0, SKL_REVID_F0);
+	pre |= IS_BXT_REVID(dev_priv, 0, BXT_REVID_B_LAST);
 
 	if (pre) {
 		DRM_ERROR("This is a pre-production stepping. "