media: v4l2-mem2mem: prevent pollerr when last_buffer_dequeued is set

If the last buffer was dequeued from the capture queue,
signal userspace. DQBUF(CAPTURE) will return -EPIPE.

But if output queue is empty and capture queue is empty,
v4l2_m2m_poll_for_data will return EPOLLERR,
This is very easy to happen in drain.

When last_buffer_dequeued is set, we shouldn't return EPOLLERR,
but return EPOLLIN | EPOLLRDNORM.

Fixes: 1698a7f1 ("media: v4l2-mem2mem: simplify poll logic")
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>