decklinkvideosink: Also stop scheduled playback when gst_element_lost_state() is...

decklinkvideosink: Also stop scheduled playback when gst_element_lost_state() is called

Unfortunately this does not go through the normal state change
machinery, so we don't get notified about this in change_state().
However we need to stop scheduled playback, so that once PLAYING is
reached again we can start scheduled playback with the correct time.

Without this, flushing seeks in PLAYING will not work correctly:
decklinkvideosink will wait before showing the new frames for the amount
of time the pipeline was in PLAYING before.