drm/i915: Disable WC PTE updates to w/a buggy IOMMU on ILK

Whilst IOMMU is enabled for the Intel GPU on Ironlake, it appears that
using WC writes to update the PTE on the GPU fails miserably. The
result looks like the majority of the writes do not land leading to
lots of screen corruption and a hard system hang.

v2: s/</<=/ to preserve the current exclusion of Sandybridge

Reported-by: Nathan Myers <ncm@cantrip.org>
Bugzilla: https://bugzilla.freedesktop.org/show_bug.cgi?id=60391
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Nathan Myers <ncm@cantrip.org>
[danvet: Remove cc: stable and add tested-by.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>