intel-iommu: Detect DMAR in hyperspace at probe time.

Many BIOSes will lie to us about the existence of an IOMMU, and claim
that there is one at an address which actually returns all 0xFF.

We need to detect this early, so that we know we don't have a viable
IOMMU and can set up swiotlb before it's too late.

Cc: stable@kernel.org
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>