dock:: fix trash problem.

the trash won't be shown on classic/efficient mode after display mode switched.

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/43d259218c8940a4aa82b196a700a5f2

Change-Id: I87a67f621dda8fc0fc2cb30f5d85fb71bb0d7f0f