x86/PCI: Add to_pci_sysdata() helper

Various helpers need the pci_sysdata just to dereference a single field in
it.  Add a little helper that returns the properly typed sysdata pointer to
require a little less boilerplate code.

[jonathan.derrick: to_pci_sysdata const argument]
Link: https://lore.kernel.org/r/1579613871-301529-2-git-send-email-jonathan.derrick@intel.com


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>