PCI/ATS: Remove unused PRI and PASID stubs

The following functions are only used by amd_iommu.c and intel-iommu.c
(when CONFIG_INTEL_IOMMU_SVM is enabled).  CONFIG_PCI_PRI and
CONFIG_PCI_PASID are always defined in those cases, so there's no need for
the stubs.

  pci_enable_pri()
  pci_disable_pri()
  pci_reset_pri()
  pci_prg_resp_pasid_required()
  pci_enable_pasid()
  pci_disable_pasid()

Remove the unused stubs.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>