ALSA: hda/realtek - Fix initialization of input amps in output paths

When initializing the output paths, we assumed the input amps have
almost two inputs blindly.  It's not only generic but even incorrect
for some codecs like ALC268 & co.  Also, the same assumption (two
sources) exists for the bind input-amp controls.

This patch changes the codes in these places to handle the input
connections in a more generic way.

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