[ALSA] hda: Dynamically create digital gain mixers

Dynamically create digital gain mixers for dmics that have out-amp
support. Also some 92HD73xx's codecs don't have DMIC gains, so this also
prevents creating dead mixers.

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