nak: Use s2r for SV_CLOCK on Kepler

cs2r is new starting with Maxwell.  Prior to that we need to use s2r
which still works just fine, it's just slower.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34619>

src/nouveau/compiler/nak/from_nir.rs

@@ -2982,7 +2982,9 @@ impl<'a> ShaderFromNir<'a> {
                 );
                 let comps = intrin.def.bit_size / 32;
                 let dst = b.alloc_ssa(RegFile::GPR, comps);
-                if idx == NAK_SV_CLOCK || idx == NAK_SV_CLOCK + 1 {
+                if self.sm.sm() >= 50
+                    && (idx == NAK_SV_CLOCK || idx == NAK_SV_CLOCK + 1)
+                {
                     debug_assert!(idx + comps <= NAK_SV_CLOCK + 2);
                     b.push_op(OpCS2R {
                         dst: dst.into(),