Na verdade, eu consertei meu problema simplesmente atualizando do Ubuntu 14.04 para o 15.04 e agora meu DP foi detectado corretamente. O kernel era muito antigo ou os drivers não estavam atualizados.
$ xrandr
Screen 0: minimum 8 x 8, current 5760 x 1080, maximum 32767 x 32767
eDP1 connected 1920x1080+1920+0 (normal left inverted right x axis y axis) 309mm x 174mm
1920x1080 60.0*+ 59.9 48.0
1680x1050 60.0 59.9
1600x1024 60.2
1400x1050 60.0
1280x1024 60.0
1440x900 59.9
1280x960 60.0
1360x768 59.8 60.0
1152x864 60.0
1024x768 60.0
800x600 60.3 56.2
640x480 59.9
DP1 disconnected (normal left inverted right x axis y axis)
DP1-1 disconnected (normal left inverted right x axis y axis)
DP1-1-1 connected 1920x1080+3840+0 (normal left inverted right x axis y axis) 527mm x 296mm
1920x1080 60.0*+ 50.0 59.9
1600x1200 60.0
1600x900 60.0
1280x1024 75.0 60.0
1152x864 75.0
1280x720 60.0 50.0 59.9
1024x768 75.1 60.0
800x600 75.0 60.3
720x576 50.0
720x480 60.0 59.9
640x480 75.0 60.0 59.9
720x400 70.1
DP1-1-2 disconnected (normal left inverted right x axis y axis)
DP1-2 disconnected (normal left inverted right x axis y axis)
DP1-2-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 527mm x 296mm
1920x1080 60.0*+ 50.0 59.9
1600x1200 60.0
1600x900 60.0
1280x1024 75.0 60.0
1152x864 75.0
1280x720 60.0 50.0 59.9
1024x768 75.1 60.0
800x600 75.0 60.3
720x576 50.0
720x480 60.0 59.9
640x480 75.0 60.0 59.9
720x400 70.1
DP1-2-2 disconnected (normal left inverted right x axis y axis)
DP1-3 disconnected (normal left inverted right x axis y axis)
DP2 disconnected (normal left inverted right x axis y axis)
HDMI1 disconnected (normal left inverted right x axis y axis)
HDMI2 disconnected (normal left inverted right x axis y axis)
VIRTUAL1 disconnected (normal left inverted right x axis y axis)
É assim que a saída do xrandr i estava esperando. Espero que isso ajude alguém se estiver enfrentando o mesmo problema com a mesma distribuição.