i965: Enable GL_ARB_ES3_1_compatibility on Gen8+ if CS are available.
i965: Enable GL_ARB_ES3_1_compatibility on Gen8+ if CS are available.

There are almost no tests in any test suite, but what little I've found
seems to work.  Ilia believes everything is in place.

v2: Predicate the enable on ES 3.1 being available (Gen8+) and also
    ARB_compute_shader being available (requested by Ilia).

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Reviewed-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler