[media] v4l2-mem2mem: support events in v4l2_m2m_poll

v4l2_m2m_poll didn't support events, but that's essential if you want to
be able to use control events for example.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>