PCI/ATS: Handle sharing of PF PRI Capability with all VFs

Per PCIe r5.0, sec 9.3.7.11, VFs must not implement the PRI Capability.  If
the PF implements PRI, it is shared by the VFs.  Since VFs don't have a PRI
Capability, pci_enable_pri() always failed, which caused IOMMU setup to
fail.

Update the PRI interfaces so for VFs they reflect the state of the PF PRI.

[bhelgaas: rebase without pri_cap caching, commit log]
Suggested-by: Ashok Raj <ashok.raj@intel.com>
Link: https://lore.kernel.org/r/b971e31f8695980da8e4a7f93e3b6a3edba3edaa.1567029860.git.sathyanarayanan.kuppuswamy@linux.intel.com
Link: https://lore.kernel.org/r/20190905193146.90250-2-helgaas@kernel.org
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Keith Busch <keith.busch@intel.com>