MdeModulePkg: Fix OptionROM scanning

The Option ROM scanner can't work as enumeration was done by the
first stage bootloader. Running it will disable the ability of the
PCIPlatform code to scan for ROMs.

Required for the following patch that enables custom Option ROM
scanning using gPciPlatformProtocol.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>

MdeModulePkg/Bus/Pci/PciBusDxe/PciEnumeratorSupport.c

@@ -2551,10 +2551,12 @@ PciEnumeratorLight (
       //
       RemoveRejectedPciDevices (RootBridgeDev->Handle, RootBridgeDev);
 
-      //
-      // Process option rom light
-      //
-      ProcessOptionRomLight (RootBridgeDev);
+      if (!PcdGetBool (PcdPciDisableBusEnumeration)) {
+        //
+        // Process option rom light
+        //
+        ProcessOptionRomLight (RootBridgeDev);
+      }
 
       //
       // Determine attributes for all devices under this root bridge