nouveau: implement Default for Push

By convention, a struct with a `new()` method which has no parameters
should have a `Default` impl which calls `new()`.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34372>

src/nouveau/headers/nv_push_rs/lib.rs

@@ -213,6 +213,12 @@ impl Push {
     }
 }
 
+impl Default for Push {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl std::ops::Deref for Push {
     type Target = [u32];