ALSA: hda - Keep autocfg.input idx value in imux table

Since the imux table entries can be a subset of autocfg.input table,
the indices of these aren't always same.  For passing the proper index
value of autocfg.input at creating input ctl labels (via
snd_hda_autocfg_input_label()), keep the corresponding autocfg.input
idx value in the index field of each imux item, which isn't used in
the generic driver.

Also, this makes easier to check the invalid imux pin for stereo mix.

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