MdeModulePkg XhciPei: Minor refinement about IoMmu

1. Call IoMmuInit() after locating gPeiUsbControllerPpiGuid.
2. Call XhcPeiFreeSched() to do cleanup in XhcEndOfPei.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

MdeModulePkg/Bus/Pci/XhciPei/XhcPeim.c

@@ -1427,6 +1427,8 @@ XhcEndOfPei (
 
   XhcPeiHaltHC (Xhc, XHC_GENERIC_TIMEOUT);
 
+  XhcPeiFreeSched (Xhc);
+
   return EFI_SUCCESS;
 }
 
@@ -1461,8 +1463,6 @@ XhcPeimEntry (
     return EFI_SUCCESS;
   }
 
-  IoMmuInit ();
-
   Status = PeiServicesLocatePpi (
              &gPeiUsbControllerPpiGuid,
              0,
@@ -1473,6 +1473,8 @@ XhcPeimEntry (
     return EFI_UNSUPPORTED;
   }
 
+  IoMmuInit ();
+
   Index = 0;
   while (TRUE) {
     Status = UsbControllerPpi->GetUsbController (