MdeModulePkg PiSmmCore: Use SxDispatch in SmmEndOfDxeHandler()

MdeModulePkg PiSmmCore: Use SxDispatch in SmmEndOfDxeHandler()

As some implementation of SMM Child Dispatcher (including SxDispatch)
may deny the handler registration after SmmReadyToLock, using
SxDispatch in SmmReadyToBootHandler() will be too late.

This patch updates code to use SxDispatch in SmmEndOfDxeHandler()
instead of SmmReadyToBootHandler().

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>