i965: Pretend there are 4 subslices for compute shader threads on Gen9+.

i965: Pretend there are 4 subslices for compute shader threads on Gen9+.

Similar to what we did for pixel shader threads - see gen_device_info.c.

We don't want to bump the actual Maximum Number of Threads though, so
we adjust it here.  For pixel shaders, we don't use max_wm_threads, so
we could just bump it globally.

Supposedly fixes Piglit tests:
arb_gpu_shader_int64/execution/built-in-functions/cs-op-div-i64vec3-int64_t
arb_gpu_shader_int64/execution/built-in-functions/cs-op-div-i64vec4-int64_t
arb_gpu_shader_int64/execution/built-in-functions/cs-op-div-u64vec4-uint64_t

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>