ACPI: PCI: combine lookup and derive

This folds acpi_pci_irq_derive() into acpi_pci_irq_lookup() so it
can be easily used by both acpi_pci_irq_enable() and acpi_pci_irq_disable().

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>