iio: buffer-dma: Get rid of outgoing queue

iio: buffer-dma: Get rid of outgoing queue

The buffer-dma code was using two queues, incoming and outgoing, to
manage the state of the blocks in use.

While this totally works, it adds some complexity to the code,
especially since the code only manages 2 blocks. It is much easier to
just check each block's state manually, and keep a counter for the next
block to dequeue.

Since the new DMABUF based API wouldn't use the outgoing queue anyway,
getting rid of it now makes the upcoming changes simpler.

With this change, the IIO_BLOCK_STATE_DEQUEUED is now useless, and can
be removed.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20231219175009.65482-2-paul@crapouillou.net
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>