decklinkvideosrc: Don't report that we have signal until we know for sure

Previously we would've reported that there is signal unless we know for sure that we don't have signal. For example signal would've been reported before the device is even opened.

Now keep track whether the signal state is unknown or not and report no signal if we don't know yet. As before, only send an INFO message about signal recovery if we actually had a signal loss before.