https://bugs.freedesktop.org/show_bug.cgi?id=69694
--- Comment #14 from Marco mpiazza@gmail.com --- I found a workaround!
Disabling second GPU in xorg.conf let me enter in X with acceleration enabled:
marco@albireo:~$ glxinfo | grep -i opengl OpenGL vendor string: X.Org OpenGL renderer string: Gallium 0.4 on AMD KABINI OpenGL version string: 2.1 Mesa 9.3.0-devel OpenGL shading language version string: 1.30
Finally!!! And all is working fine.
Then the bug is related to having 2 GPU. At least now I can play games (with only one GPU).