pan/midgard: Implement barriers

pan/midgard: Implement barriers

Barriers execute on the texture pipeline on Midgard, so let's
tentatively handle barrier() as conservatively as possible (forcing
memory barriers of both buffers and shared memory). Implementation isn't
quite there yet -- it doesn't look at interactions of adjacent barriers
like it's supposed to -- but the core is there.

Fixes dEQP-GLES31.functional.compute.basic.ssbo_local_barrier_single_invocation

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3835>