OK, finally got some good news. I'm rebuilding the KMP again with a patch to add a module option "aux_backlight" instead of blindly disabling the aux backlight. So keep your working KMP for now, and check later the new KMP and test with amdgpu.aux_backlight=0 boot option. This is no solution but an acceptable workaround, I suppose. I'm going to submit two patches to upstream. Oh, BTW, which value was shown in the "XXX ext_caps=..." line?