intel-iommu: Export a flag indicating that the IOMMU is used for iGFX.

We really don't want this to work in the general case; device drivers
*shouldn't* care whether they are behind an IOMMU or not. But the
integrated graphics is a special case, because the IOMMU and the GTT are
all kind of smashed into one and generally horrifically buggy, so it's
reasonable for the graphics driver to want to know when the IOMMU is
active for the graphics hardware.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>