SourceLevelDebugPkg/SmmDebugAgent: mMailboxPointer is used before set

SourceLevelDebugPkg/SmmDebugAgent: mMailboxPointer is used before set

mMailboxPointer is used before it is initialization. This issue only happens
when SmmDebugAgent is initialized without PeiDebugAgent or DxeDebugAgent
initialization. The fix is to use Mailbox instead.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>