There is probably a bug in dither.depth property set cause its value compared with dither.mode:

//nouveau_connector.c
 
if (property == disp->dithering_mode) {
    if (asyc->dither.mode != val) {
        asyc->dither.mode = val;
        asyc->set.dither = true;
    }
} else
if (property == disp->dithering_depth) {
    if (asyc->dither.mode != val) { //Should be dither.depth
        asyc->dither.depth = val;
        asyc->set.dither = true; //triggers dithering regardless of dither.mode set above. Copy/Paste from above? Should be removed?
    }
}