[media] v4l2-event: add optional merge and replace callbacks

When the event queue for a subscribed event is full, then the oldest
event is dropped. It would be nice if the contents of that oldest
event could be merged with the next-oldest. That way no information is
lost, only intermediate steps are lost.

This patch adds optional replace() (called when only one kevent was allocated)
and merge() (called when more than one kevent was allocated) callbacks that
will be called to do this job.

These two callbacks are implemented for the V4L2_EVENT_CTRL event.

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