iommu/vt-d: Take CONFIG_PCI_ATS into account

pci_dev::physfn is only available when CONFIG_PCI_ATS is set. The recent
fix for the irqdomain rework missed that dependency which makes the build
fail when CONFIG_PCI_ATS=n.

Add the necessary #ifdeffery.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: ff828729 ("iommu/vt-d: Cure VF irqdomain hickup")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Joerg Roedel <joro@8bytes.org>