IntelSiliconPkg-Vtd: Code Optimization

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1770



1) DisableDMAr Function Code Optimization
Optimize the flow to follow the VT-d spec requirements.

2) Renamed InitDmar() to InitGlobalVtd()
The oringal function name is misleading

Signed-off-by: Evelyn Wang <iwen.evelyn.wang@intel.com>
Cc: Jenny Huang <jenny.huang@intel.com>
Cc: More Shih <more.shih@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>