media: v4l2-mem2mem: add q->error check to v4l2_m2m_poll()

The v4l2_m2m_poll function didn't check whether q->error
was set for either of the two queues. Add support for this.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>