x86/PCI: VMD: Initialize list item in IRQ disable

Multiple calls to disable an IRQ would have caused the driver to
dereference a poisoned list item.  This re-initializes the list to allow
multiple requests to disable the IRQ.

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