PCI: add pci_request_acs

Commit ae21ee65e8 "PCI: acs p2p upsteram
forwarding enabling" doesn't actually enable ACS.

Add a function to pci core to allow an IOMMU to request that ACS
be enabled.  The existing mechanism of using iommu_found() in the pci
core to know when ACS should be enabled doesn't actually work due to
initialization order;  iommu has only been detected not initialized.

Have Intel and AMD IOMMUs request ACS, and Xen does as well during early
init of dom0.

Cc: Allen Kay <allen.m.kay@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

arch/x86/kernel/amd_iommu_init.c

@@ -1330,6 +1330,8 @@ void __init amd_iommu_detect(void)
 		gart_iommu_aperture_disabled = 1;
 		gart_iommu_aperture = 0;
 #endif
+		/* Make sure ACS will be enabled */
+		pci_request_acs();
 	}
 }