MdeModulePkg/PciBusDxe: Install PciEnumerationComplete after PciIo

Per PI spec, the PciEnumerationComplete protocol installation
should be after PciIo installation.
Today's implementation installs the PciEnumerationComplete
after hardware enumeration is completed, but before PciIo
installation.
The change corrects the spec/implementation gap.
The change also benefits certain implementation that depends on
the PciIo handle in PciEnumerationComplete callback.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>

MdeModulePkg/Bus/Pci/PciBusDxe/PciBus.c

@@ -360,8 +360,19 @@ PciBusDriverBindingStart (
   //
   StartPciDevices (Controller);
 
-  gFullEnumeration = FALSE;
-  return EFI_SUCCESS;
+  if (gFullEnumeration) {
+    gFullEnumeration = FALSE;
+
+    Status = gBS->InstallProtocolInterface (
+                    &PciRootBridgeIo->ParentHandle,
+                    &gEfiPciEnumerationCompleteProtocolGuid,
+                    EFI_NATIVE_INTERFACE,
+                    NULL
+                    );
+    ASSERT_EFI_ERROR (Status);
+  }
+
+  return Status;
 }
 
 /**