drm/i915: Add WaInsertDummyPushConstP for bxt and kbl

Add this workaround for both bxt and kbl up to until
rev B0.

References: HSD#2136703
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-16-git-send-email-mika.kuoppala@intel.com