ALSA: hda - Add support for HP dv6

Add the quirk entry for HP dv6.  Also add a workaround for the headphone
detection by setting hp_detect=1 beforehand.  Without this, the driver
won't do auto-muting because BIOS doesn't give any HP pin but only a
line-out pin.

Signed-off-by: Takashi Iwai <tiwai@suse.de>