x86/PCI: VMD: Eliminate index member from IRQ list

Use math to discover the IRQ list index number relative to the IRQ list
head.

Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Keith Busch <keith.busch@intel.com>