asahi: optimize out empty dispatches

this occurs with GS lowering.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34638>

src/gallium/drivers/asahi/agx_state.c

@@ -5297,6 +5297,8 @@ agx_launch(struct agx_batch *batch, struct agx_grid grid,
            unsigned variable_shared_mem)
 {
    struct agx_context *ctx = batch->ctx;
+   if (!linked && agx_is_shader_empty(&cs->b))
+      return;
 
    /* To implement load_num_workgroups, the number of workgroups needs to be
     * available in GPU memory. This is either the indirect buffer, or just a