intel-iommu: Force-disable IOMMU for iGFX on broken Cantiga revisions.

intel-iommu: Force-disable IOMMU for iGFX on broken Cantiga revisions.

commit 2d9e667efdfb4e986074d98e7d9a424003c7c43b upstream.

Certain revisions of this chipset appear to be broken. There is a shadow
GTT which mirrors the real GTT but contains pre-translated physical
addresses, for performance reasons. When a GTT update happens, the
translations are done once and the resulting physical addresses written
back to the shadow GTT.

Except sometimes, the physical address is actually written back to the
_real_ GTT, not the shadow GTT. Thus we start to see faults when that
physical address is fed through translation again.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>