i965/cs: Enable barrier in MEDIA_INTERFACE_DESCRIPTOR

i965/cs: Enable barrier in MEDIA_INTERFACE_DESCRIPTOR

Enable barrier in MEDIA_INTERFACE_DESCRIPTOR if the program uses the
barrier() GLSL function.

On Ivy Bridge and Haswell, this allows the piglit test
tests/spec/arb_compute_shader/execution/simple-barrier-atomics.shader_test
to pass. On gen8, this enables a similar test with a local group size
of 896 to pass.

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>