I have rx 560 on Ubuntu 16.04. Steam games worked on Mesa driver, after I installed amdgpu-pro 17.50 (and I added users to the video groub) steam starts with this: Error OpenGL GLX context is not using direct rendering, which may cause performance problems. When I try to start for example CS:GO error raise: Could not required OpenGL entry point "glGetError"! Either your video card is unsupported, or your OpenGL driver needs to be updated.
