avcodec/v4l2_buffers: don't prevent enqueue capture buffer to driver

avcodec/v4l2_buffers: don't prevent enqueue capture buffer to driver

Enqueue/dequeue of the capture buffers should continue while draining.

Reference: linux/Documentation/media/uapi/v4l/dev-decoder.rst

    "The client must continue to handle both queues independently,
     similarly to normal decode operation. This includes:
     ...
     - queuing and dequeuing CAPTURE buffers, until a buffer marked with
       the V4L2_BUF_FLAG_LAST flag is dequeued"

Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>