res:dock:: invoke too much dbus method.

the GetDisplayMode is invoked when update_time.

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/6fdeaf2da56f4a59a06fdcdfebd05179/

Change-Id: I09e0c8004a3830bd385ec7e3082011c8b5d6aeb6