mesa: use _mesa_has_compute_shaders instead of extension check
mesa: use _mesa_has_compute_shaders instead of extension check

This was really the original purpose, for enabling the path for
ES3.1 tests without the extension being set. Set also fallthrough
comment for Coverity (caught by Matt).

v2: .. and test the right way, not wrong one (Ilia Mirkin)

Signed-off-by: Tapani Pälli <cdcb67a5e49297de94340ca09f25994bd5e7f762@intel.com>
Reviewed-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler