sink-input, source-output: remove the state getters

sink-input, source-output: remove the state getters

pa_sink_input_get_state() and pa_source_output_get_state() just return
the state variable. We can as well access the state variable directly.

There are no behaviour changes, except that some filter sources accessed
the main thread's state variable from their push() callbacks. I fixed
them so that they use the thread_info.state variable instead.