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
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>