decklinksink: Wait for the pipeline clock instead of the decklink clock

decklinksink: Wait for the pipeline clock instead of the decklink clock

Otherwise we're going to starve other elements if the decklink clock
is slower than the pipeline clock, or starts much later.

Of course this will still cause problems if the decklink clock and ours are
completely out of sync, or running at a very different rate. But this at least
works better now.