s3tc doesn't work when using the radeonsi driver.

Example:

kallisti5@eris ~ :) $ find /usr/lib -name "radeonsi*"
/usr/lib/xorg/modules/dri/radeonsi_dri.so

kallisti5@eris ~ :) $ glxinfo | grep renderer
OpenGL renderer string: Gallium 0.4 on AMD TAHITI

No EXT_texture_compress (aka s3tc)
kallisti5@eris ~ :) $ glxinfo | grep EXT_texture_compress

Move radeonsi driver somewhere else forcing fallback to swpipe:

kallisti5@eris ~ :( $ sudo mv /usr/lib/xorg/modules/dri/radeonsi_dri.so /tmp/

Now using llvmpipe:
kallisti5@eris ~ :) $ glxinfo | grep renderer
libGL error: failed to load driver: radeonsi
libGL error: Try again with LIBGL_DEBUG=verbose for more details.
OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 3.3, 128 bits)

s3tc now working:
kallisti5@eris ~ :) $ glxinfo | grep EXT_texture_compress
libGL error: failed to load driver: radeonsi
libGL error: Try again with LIBGL_DEBUG=verbose for more details.
    GL_EXT_texture_array, GL_EXT_texture_compression_dxt1, 
    GL_EXT_texture_compression_latc, GL_EXT_texture_compression_rgtc, 
    GL_EXT_texture_compression_s3tc, GL_EXT_texture_cube_map, 
kallisti5@eris ~ :) $