SourceLevelDebugPkg: Avoid to re-init IDT table again at SMI entry

SourceLevelDebugPkg: Avoid to re-init IDT table again at SMI entry

Current SmmDebugAgentLib will initialize IDT table to support source debugging
at each time SMI entry on SMM BSP. Actually, we only need to initialize IDT
table at first time SMI entry.

Add one flag to avoid re-initializing IDT table.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@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>