ALSA: hda - Consolidate default automute functions for Realtek

There are two entry points for the headphone automute functions for
Realtek, alc_automute_amp() and alc_automute_pin().  These call the
same function in the end, so we can basically consolidate these
with a flag in spec.
Signed-off-by: Takashi Iwai <tiwai@suse.de>