combine-sink: Fix unsafe message handling

combine-sink: Fix unsafe message handling

This fixes a crash. sink_input_pop_cb() drains the message queue that receives
memchunks from the combine sink thread to avoid requesting more audio too soon.
The same message queue received also SET_REQUESTED_LATENCY messages, which
generate rewind requests. Rewind requests shouldn't be issued in the pop()
callback, doing so results in an assertion error. Therefore, it was not safe to
drain the message queue in the pop() callback, but usually the queue is empty,
so this bug was not immediately detected.

This patch splits the message queue into two queues: audio_inq and control_inq.
audio_inq receives only messages containing memchunks, and control_inq receives
only the SET_REQUESTED_LATENCY messages. The pop() callback only drains the
audio queue, which avoids the rewind requests in the pop() callback.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=90489