PCI: Remove pci_ats_enabled()

Remove pci_ats_enabled().  There are no callers outside the ATS code
itself.  We don't need to check ats_cap, because if we don't find an ATS
capability, we'll never set ats_enabled.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Joerg Roedel <jroedel@suse.de>