st/mesa: do not init limits when compute shaders are not supported

st/mesa: do not init limits when compute shaders are not supported

When the number of uniform blocks is less than 12,
ARB_uniform_buffer_object can't be enabled and the maximum GL version
is not even 3.1...

This fixes a regression introduced in 7c79c1e (st/mesa: add compute
shader state) if the maximum number of uniform blocks allowed for
compute shaders is less than 12. This happens on Kepler but this might
also affect other Gallium drivers.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reported-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Tested-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>