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>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 69150a0..2ce249d 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -2502,6 +2502,7 @@
 #define SKL_REVID_F0		0x5
 
 #define BXT_REVID_A0		0x0
+#define BXT_REVID_A1		0x1
 #define BXT_REVID_B0		0x3
 #define BXT_REVID_C0		0x9