MdeModulePkg/VarCheckPolicyLib: Fix buffer valid check for MM

For MM, the MM Handlers do not need to validate the buffer if it is
the CommBuffer passed from MmCore through the MmiHandler() parameter.
Return TRUE directly in this case. Fix buffer valid check for MM in
this patch.

Signed-off-by: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Hongbin1 Zhang <hongbin1.zhang@intel.com>
Cc: Wei6 Xu <wei6.xu@intel.com>
Cc: Dun Tan <dun.tan@intel.com>
Cc: Yuanhao Xie <yuanhao.xie@intel.com>

MdeModulePkg/Library/VarCheckPolicyLib/VarCheckPolicyLibStandaloneMm.c

@@ -47,5 +47,5 @@ VarCheckPolicyIsPrimaryBufferValid (
   IN UINT64                Length
   )
 {
-  return MmIsBufferOutsideMmValid (Buffer, Length);
+  return TRUE;
 }