winegstreamer: Use WG_MAJOR_TYPE_UNKNOWN to mark a stream that should not be forced into a certain format.

Instead of checking the enabled state.

The reasoning here is that we would like to keep the stream enabled—that is,
buffer samples instead of discarding them—even when the client is not yet
reading samples from it, so that we can avoid wasting CPU power in decoding.
Signed-off-by: Zebediah Figura <zfigura@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>