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>

src/intel/compiler/brw_fs.h

@@ -315,6 +315,7 @@ public:
     */
    int *push_constant_loc;
 
+   fs_reg thread_local_id;
    fs_reg frag_depth;
    fs_reg frag_stencil;
    fs_reg sample_mask;