ALSA: hda - Introduce snd_hda_codec_amp_init*()

The new function snd_hda_codec_amp_init() (and the stereo variant)
initializes the amp value only once at the first access.  If the amp
was already initialized or updated, this won't do anything more.

It's useful for initializing the input amps that are in the part of
the path but never used.

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