[ALSA] hda: Add dynamic mono mixer support for STAC92xx codecs

Allows for dynamically creating mono out mixer controls and well
as mono mux controls.

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