intel/cs: Push subgroup ID instead of base thread ID

intel/cs: Push subgroup ID instead of base thread ID

We're going to want subgroup ID for SPIR-V subgroups eventually anyway.
We really only want to push one and calculate the other from it.  It
makes a bit more sense to push the subgroup ID because it's simpler to
calculate and because it's a real API thing.  The only advantage to
pushing the base thread ID is to avoid a single SHL in the shader.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>