[media] adv7604/adv7842: replace FMT_CHANGED by V4L2_DEVICE_NOTIFY_EVENT

This makes it easier for the bridge driver to just passthrough such
events to the corresponding device node.

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