gallium: move _vbo_DestroyContext() call

Call it before freeing core Mesa state to avoid references to freed buffer objects.

src/mesa/state_tracker/st_context.c

@@ -188,8 +188,6 @@ static void st_destroy_context_priv( struct st_context *st )
    st_destroy_drawtex(st);
 #endif
 
-   _vbo_DestroyContext(st->ctx);
-
    for (i = 0; i < Elements(st->state.sampler_texture); i++) {
       pipe_texture_reference(&st->state.sampler_texture[i], NULL);
    }
@@ -223,6 +221,8 @@ void st_destroy_context( struct st_context *st )
 
    _mesa_delete_program_cache(st->ctx, st->pixel_xfer.cache);
 
+   _vbo_DestroyContext(st->ctx);
+
    _mesa_free_context_data(ctx);
 
    st_destroy_context_priv(st);