softpipe: add support for compute shaders. (v2)

softpipe: add support for compute shaders. (v2)

This enables ARB_compute_shader on softpipe. I've only
tested this with piglit so far, and I hopefully plan
on integrating it with my vulkan work. I'll get to
testing it with deqp more later.

The basic premise is to create up to 1024 restartable
TGSI machines, and execute workgroups of those machines.

v1.1: free machines.
v2: deqp fixes - add samplers support, finish
atomic operations, fix load/store writemasks.

Acked-by: Roland Scheidegger <sroland@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>