ALSA: HDA VIA: Add low current mode for power saving.

For VT1708B, VT1708S and VT1702, enter low current mode if no analog
stream is opened and all aa path mute.

Signed-off-by: Lydia Wang <lydiawang@viatech.com.cn>
Signed-off-by: Logan Li <loganli@viatech.com.cn>
Signed-off-by: Takashi Iwai <tiwai@suse.de>