Comment # 10 on bug 940717 from Takashi Iwai

Could you try to create a patch firmware file containing the following in
/lib/firmware/alsa/pincfg

[codec]
0x10ec0269 0x102802da 0
[pincfg]
0x1b 0x0121402f

... and create /etc/modprobe.d/50-hda.conf containing the following line:

options snd-hda-intel patch=alsa/pincfg

This will remap the rear output as a headphone, and both front and rear
headphones will use the DAC 0x03 while the speaker is mapped to DAC 0x02.
If it still doesn't work, please give alsa-info.sh output for verification.