IntelSiliconPkg VTdPmrPei: Return SUCCESS when Mapping == NULL in Unmap

NULL is returned to Mapping when Operation is BusMasterCommonBuffer or
BusMasterCommonBuffer64 in PeiIoMmuMap().
So Mapping == NULL is valid when calling PeiIoMmuUnmap().

940dbd071e9f01717236af236740aa0da716805f wrongly changed EFI_SUCCESS
to EFI_INVALID_PARAMETER when Mapping == NULL in PeiIoMmuUnmap().
This patch is to correct it.

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>