[media] v4l2-dv-timings: add callback to handle exceptions

In most cases the v4l2_bt_timings_cap struct has all the information
necessary to determine valid timings, but occasionally there are exceptions.
Add a callback function to be able to test for those exceptions.

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