intel/iommu: force writebuffer-flush quirk on Gen 4 Chipsets

We already have the quirk entry for the mobile platform, but also
reports on some desktop versions. So be paranoid and set it
everywhere.

References: http://www.mail-archive.com/dri-devel@lists.freedesktop.org/msg33138.html
Cc: stable@vger.kernel.org
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: "Sankaran, Rajesh" <rajesh.sankaran@intel.com>
Reported-and-tested-by: Mihai Moldovan <ionic@ionic.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>