drm/i915/kbl: Add WaClearSlmSpaceAtContextSwitch

This workaround for bdw and chv, is also needed for kbl A0.

References: HSD#1911519, BSID#569
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-24-git-send-email-mika.kuoppala@intel.com