nouveau: fix max_warps_per_mp_for_sm for builds with asserts disabled

Fixes: 1b756be2bca ("nvk: properly calculate SLM region by taking per arch limits into account")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24326>

src/nouveau/winsys/nouveau_device.c

@@ -109,6 +109,8 @@ max_warps_per_mp_for_sm(uint8_t sm)
       return 64;
    default:
       assert(!"unkown SM version");
+      // return the biggest known value
+      return 64;
    }
 }