ALSA: VIA HDA: Mute/unmute mixer conncted to Headphone for VT1718S.

When switch HP independent mode, mute/unmute connctions of mixer  which is
connected to headphone for VT1718S.

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