intel-iommu: Check for 'DMAR at zero' BIOS error earlier.

Chris Wright has some patches which let us fall back to swiotlb nicely
if IOMMU initialisation fails. But those are a bit much for 2.6.32.

Instead, let's shift the check for the biggest problem, the HP and Acer
BIOS bug which reports a DMAR at physical address zero. That one can
actually be checked much earlier -- before we even admit to having
detected an IOMMU in the first place. So the swiotlb init goes ahead as
we want.

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