ALSA: hda - Fix click noise at start on Dell XPS13

Dell XPS13 produces a click noise at boot up, and Gabriele spotted out
that it's triggered by the initial pin control of the mic (NID 0x19).
This has to be set to Hi-Z Vref while the driver initializes to Vref
80% as a normal mic.

This patch fixes the generic parser code not to override the target
vref if it has been already set by the driver, and adds a proper
initialization of the target vref for this pin in the Realtek driver
side.

Reported-and-tested-by: Gabriele Mazzotta <gabriele.mzt@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>