pan/bi: Fix atomic exchange on Valhall

Copypaste fail when switching to unified atomics, missed becuase I don't have
any Valhall hardware and Valhall isn't in CI. (Good news, that means it probably
didn't affect anyone in the mean time :-p)

Fixes crashes with lots of dEQP-GLES31 tests observed under drm-shim.

Fixes: e258083e07 ("pan/bi: Use unified atomics")
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23120>

src/panfrost/compiler/bifrost_compile.c

@@ -1632,7 +1632,7 @@ bi_emit_intrinsic(bi_builder *b, nir_intrinsic_instr *instr)
 
       if (op == nir_atomic_op_xchg) {
          bi_emit_axchg_to(b, dst, bi_src_index(&instr->src[0]), &instr->src[1],
-                          BI_SEG_WLS);
+                          BI_SEG_NONE);
       } else {
          assert(nir_src_bit_size(instr->src[1]) == 32);