MdeModulePkg/DisplayEngineDxe: Remove unnecessary parentheses in 'if'

This commit removes unnecessary parentheses in 'if' statements to
resolve the build failures by the XCODE5 tool chain.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdeModulePkg/Universal/DisplayEngineDxe/Popup.c

@@ -686,7 +686,7 @@ CreatePopup (
   }
 
   gMessageString = HiiGetString (HiiHandle, Message, NULL);
-  if((gMessageString == NULL)) {
+  if(gMessageString == NULL) {
     return EFI_INVALID_PARAMETER;
   }