ALSA: HDA - Add Independent Headphone for all models of ad1988/ad1989

- Add "AD198x Headphone" playback device for independent headphone playback
  while playing 7.1 surround using rear panel audio jacks.

- Remove "6stack-dig-fp" model since "Headphone Playback Volume" control using
  DAC0 instead of DAC1 (HDA_FRONT) was already added to all models.

- Add "Independent HP" switch to enable/disable this playback device.
  When the switch is OFF, headphone use "copy front" mode to get the front
  channel as the green jack.
  When the switch is ON, you can play stereo sound through "AD198x Headphone"
  device to headphone while playing 7.1 surround sound through "AD198x Analog"
  device.
  The switch cannot be changed when either "AD198x Headphone" or "AD198X Analog"
  is open.

Signed-off-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>