intel/cs: Rework the way thread local ID is handled

intel/cs: Rework the way thread local ID is handled

Previously, brw_nir_lower_intrinsics added the param and then emitted a
load_uniform intrinsic to load it directly.  This commit switches things
over to use a specific NIR intrinsic for the thread id.  The one thing I
don't like about this approach is that we have to copy thread_local_id
over to the new visitor in import_uniforms.

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