If the SerialBuffers can just feed the same line queue, then we don't need
the extra threads reading line queues into a new merged line queue.
Less python threading code is always better. Plus, now we can pass args
to SerialBuffer.lines() for timeout/phase.
Acked-by: Juan A. Suarez <jasuarez@igalia.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17096>