frontend: Use simple_heads_changes for head enablement
Similarly to all the other back-ends do the same for the RDP one.
With this change the remote output will be placed, similar to the VNC/PipeWire on the right side from the native one, when both backends are loaded.
But ultimately this patch is about having all backends share the same code path.
Fixes: #820 (closed) Signed-off-by: Marius Vlad marius.vlad@collabora.com