Looks like Xonotic uses derivatives with GLSL 1.30, which radeonsi doesn't
implement yet, so it falls back to dummy shaders:

warning: failed to translate tgsi opcode DDX to LLVM
Failed to translate shader from TGSI to LLVM
EE ../../../../../src/gallium/drivers/radeonsi/si_state.c:1951 si_shader_select
- Failed to build shader variant (type=1) -22

Maybe you can work around this by disabling / lowering some settings in
Xonotic, or you could try the WIP patches from
http://lists.freedesktop.org/archives/mesa-dev/2013-June/040406.html and
http://lists.freedesktop.org/archives/mesa-dev/2013-June/040412.html .