decodebin: only emit 'drained' signal on top chain

This a proposed fix to issue #751 (closed). As I am not familiar with the inner workings, this attempt at fixing the issue might be completely wrong. However, in my understanding it should be enough to emit the drained signal when the top level chain drained, moreover, I assumed that the top level chain has no parent. (This fix fails if there are multiple top-level chains - I have not found anything that would indicate such things exist.)