Without this patch, internal speaker and line-out work,
but front headphone output jack stays silent on the
Mac Pro 4,1.

This code path also gets executed on the MacPro 5,1 due
to identical codec SSID, but i don't know if it has any
positive or adverse effects there or not.

(v2) Implement feedback from Takashi Iwai: Reuse
     alc889_fixup_mbp_vref and just add a new nid
     0x19 for the MacPro 4,1.
Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>