1. Get default value base on the priority: default opcode has highest priority and get from minimum or first option has lowest.
2. When default value saved in expression, not set default value for this question.
Signed-off-by: ydong10
Reviewed-by: lgao4
git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12738 6f19259b-4bc3-4df7-8a09-765794883524