drm/i915: implement WaForceL3Serialization on VLV and IVB

References: https://bugs.freedesktop.org/show_bug.cgi?id=50250


Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Antti Koskipää <antti.koskipaa@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>