mesa: Let compute shaders work in compatibility profiles

mesa: Let compute shaders work in compatibility profiles

The extension is already advertised in compatibility profile, but
the _mesa_has_compute_shaders only returns true in core profile.
If we advertise it, we should allow it to work.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>