virtual_str_enc_funcs omits 7 ops (NULL) — latent NULL function-pointer call if virtual encoders are ever instantiated and cross-assigned to a real DP stream by the allocator fallback

Summary

The vtable fills 12 of 19 members (setup_stereo_sync, lvds_set_stream_attribute, 5 audio ops NULL) and the encoder allocator fallback does not skip ENGINE_ID_VIRTUAL - under encoder pressure a real DP stream would get a virtual encoder and NULL-call setup_stereo_sync at dc_link.c:2545. Dead code today (num_virtual_links hardcoded 0). Complete-the-vtable fix.