greeter:handle the mult-monitors for greeter

tower:https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/3d5c5bfe7599400083ca5aae32b1fae0/

Change-Id: I8f7c7254e2d142cf2f6b4b640708c41b2795a51e