drm/i915/bxt: remove snooping workaround on old A revisions

No need to cater for old A revisions.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487172099-24873-3-git-send-email-jani.nikula@intel.com