ALSA: hda/realtek - Create individual mute switches for shared DAC

Even if the outputs are using shared DACs, we can still create individual
mute siwtches since they are assigned per pin.  This allows to create,
e.g. Speaker and Bass Speaker mute switches while the single volume is
used for these outputs.

Signed-off-by: Takashi Iwai <tiwai@suse.de>