d3d12: Fix MSAA-disabling pass; sample mask should be 0 for helper lanes

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26180>

src/gallium/drivers/d3d12/d3d12_nir_passes.c

@@ -805,7 +805,7 @@ lower_multisampling_instr(nir_builder *b, nir_instr *instr, void *_data)
    case nir_intrinsic_load_sample_id:
       return nir_imm_int(b, 0);
    case nir_intrinsic_load_sample_mask_in:
-      return nir_imm_int(b, 1);
+      return nir_b2i32(b, nir_ine_imm(b, &intr->def, 0));
    default:
       unreachable("Invalid intrinsic");
    }