drm/i915/bxt: add revision id for A1 stepping and use it

Prefer inclusive ranges for revision checks rather than "below B0". Per
specs A2 is not used, so revid <= A1 matches revid < B0.

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1445343722-3312-2-git-send-email-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>