PCI/ATS: Cache PRI Capability offset

Previously each PRI interface searched for the PRI Capability.  Cache the
capability offset the first time we use it instead of searching each time.

[bhelgaas: commit log, reorder patch to later, call pci_pri_init() from
pci_init_capabilities()]
Link: https://lore.kernel.org/r/0c5495d376faf6dbb8eb2165204c474438aaae65.156
7029860.git.sathyanarayanan.kuppuswamy@linux.intel.com
Link: https://lore.kernel.org/r/20190905193146.90250-5-helgaas@kernel.org
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>