If a mixer is "global", it is available even if the output isn't
open.  However, since the check was changed from IsEnabled() to
IsReallyEnabled(), enabled outputs have not yet been used have not
been "really" enabled yet, preventing using the mixer.

Fixes a regression by commit 35dbc1a9
(part of https://github.com/MusicPlayerDaemon/MPD/pull/1480).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1563