iommu/vt-d: Fix compile error with CONFIG_PCI_ATS not set

Fix the compile error below (CONFIG_PCI_ATS not set):

drivers/iommu/intel/dmar.c: In function ‘vf_inherit_msi_domain’:
drivers/iommu/intel/dmar.c:338:59: error: ‘struct pci_dev’ has no member named ‘physfn’; did you mean ‘is_physfn’?
  338 |  dev_set_msi_domain(&pdev->dev, dev_get_msi_domain(&pdev->physfn->dev));
      |                                                           ^~~~~~
      |                                                           is_physfn

Fixes: ff828729

 ("iommu/vt-d: Cure VF irqdomain hickup")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/linux-iommu/CAMuHMdXA7wfJovmfSH2nbAhN0cPyCiFHodTvg4a8Hm9rx5Dj-w@mail.gmail.com/
Link: https://lore.kernel.org/r/20201119055119.2862701-1-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>