[ALSA] hda: 92HD71BXX Mono Mute Support

Added a mono output mute mixer for the 92hd71bxx family of codecs, this
also removes the need for the mono out node to explicitly unmuted in the
core init.

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>