[media] vivi: let vb2_poll handle events

The vb2_poll function now tests for events and sets POLLPRI accordingly.
So there it is no longer necessary to test for it in the vivi driver.

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