libweston: Don't send output_changed signal when moving disabled outputs

weston_output_set_position() currently assumes the output is enabled, but
we could be using weston_output_move() to configure an output that hasn't
yet been enabled.

If that's the case, we don't want to send signals or perform setup that
will eventually happen when the output is enabled anyway.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>