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>