BaseTools: Enhance the *P Flag display for Structure Pcd

BaseTools: Enhance the *P Flag display for Structure Pcd

Cover the case:
1.only define the structure Pcd in DEC file, it should not have any
Flag.
2.In the DEC file and DSC file only have the PCD's default value, and
without the field value, it should have *P Flag.

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