If you only poll for events, then vb2_poll will return POLLPRI | POLLERR if
no streaming is in progress. That's not right, it's perfectly valid to
poll just for events.

Cc: Pawel Osciak <pawel@osciak.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>