BaseTools: AutoGen - change class variable to funciton variable

BaseTools: AutoGen - change class variable to funciton variable

This variable is only used in one function, make it local there.
Also when iterating on the variable, use dict.items() to get value
instead of re-looking up the value multiple times.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>