irqchip/gic-v3-its: Add handling of PCI requester id.

Replace open coded generation PCI/MSI requester id with call to the
new function pci_msi_domain_get_msi_rid() which applies the "msi-map"
to the id value.

Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>