ALSA: hda - VIA: Update VT1718S initial verbs

Add a verb to enable control amplifier of stereo mixer in VT1718S
initial verbs. Set stereo mixer default amplifier value as un-mute.

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