x86: Move pci_iommu_init to rootfs_initcall()

We want this to happen after the PCI quirks, which are now running at
the very end of the fs_initcalls.

This works around the BIOS problems which were originally addressed by
commit db8be50c ('USB: Work around BIOS
bugs by quiescing USB controllers earlier'), which was reverted in
commit d93a8f82

.

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