MdeModulePkg/ConSplitterDxe: Optimize the ConSplitterTextOutSetMode

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1412

For Console Out device, it would always set all present devices'
text out mode again through ConSplitterTextOutSetMode while adding
devices. That may cause the screen cleared for serval times.
So add a BOOLEAN to judge if it is adding device then we will not
set the same text mode again for same console out device.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael Turner <Michael.Turner@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>

MdeModulePkg/Universal/Console/ConSplitterDxe/ConSplitter.h

@@ -1,7 +1,7 @@
 /** @file
   Private data structures for the Console Splitter driver
 
-Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2019, Intel Corporation. All rights reserved.<BR>
 SPDX-License-Identifier: BSD-2-Clause-Patent
 
 **/
@@ -218,6 +218,8 @@ typedef struct {
   UINTN                                 TextOutQueryDataCount;
   INT32                                 *TextOutModeMap;
 
+  BOOLEAN                               AddingConOutDevice;
+
 } TEXT_OUT_SPLITTER_PRIVATE_DATA;
 
 #define TEXT_OUT_SPLITTER_PRIVATE_DATA_FROM_THIS(a) \