Allow ProducerBuffer gain posted buffer.

Say there are 3 buffers allocated in buffer queue or some sort of
containers of the buffers, and all of the three buffers are in
posted state. Consumers are taking their time to do other things.
In this case, the producer who is using the buffers from this buffer
container would not be able to produce image because there is no buffer
available (unless the container of the buffer decide to allocate new
buffers). This change allows the container of the buffer decide whether
to reuse an old posted buffer, or to allocate new buffer, for the
producer side of the buffer container to produce image into.

Bug: 80164475
Test: buffer_hub-test on Taimen and Vega
Change-Id: I8c3d10a3b32ffa4bbf24da176a694b12c4dc3a5d