drm/amdgpu: flush delete wq after wait fence

[why]
lru_list not empty warning in sw fini during repeated device bind unbind.
There should be a amdgpu_fence_wait_empty() before the flush_delayed_work()
call as Christian suggested.

[how]
Move to do flush_delayed_work for ttm bo delayed delete wq after fence_driver_hw_fini.

Tested by: Yiqing Yao <yiqing.yao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Yiqing Yao <yiqing.yao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>