[Intel-gfx] Enable iommu on gfx by default

[Chris Wilson]

Other than Broadwell being fubar (and Ironlake + g4x being special in
their own way), there appears to be little fallout from enabling iommu.
(The biggest open question is over performance, TLB misses are much more
expensive and that impacts meda/CL/GL throughput.) Enabling iommu/dmar
makes our CI much more powerful, instead of a random GPU write causing
memcorruption which may or may not impact the system, we get a DMAR
fault. So once and for all we will be able to ascertain whether those
sporadic memcorruption are truly our fault.
-Chris