intel-iommu: Avoid panic() for DRHD at address zero.

If the BIOS does something obviously stupid, like claiming that the
registers for the IOMMU are at physical address zero, then print a nasty
message and abort, rather than trying to set up the IOMMU and then later
panicking.

It's becoming more and more obvious that trusting this stuff to the BIOS
was a mistake.

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