iris: Set MOCS on NULL vertex buffers

We'd like to add safeguards against accidental use of MOCS 0 (uncached),
which can have large performance implications.  One case where we use
MOCS of 0 is 3DSTATE_VERTEX_BUFFERS where we set NullVertexBuffer.
It shouldn't matter here, as there's no actual buffer to be cached.

That said, it should be harmless to set MOCS for null vertex buffers.
We can assume an internal buffer and request isl's vertex buffer MOCS.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13480>

src/gallium/drivers/iris/iris_state.c

@@ -3477,6 +3477,8 @@ iris_set_vertex_buffers(struct pipe_context *ctx,
 #endif
          } else {
             vb.NullVertexBuffer = true;
+            vb.MOCS = iris_mocs(NULL, &screen->isl_dev,
+                                ISL_SURF_USAGE_VERTEX_BUFFER_BIT);
          }
       }
    }