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

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>