Remove the TUI register window rerender overload

After these restructurings, it should be clear that the rerender
overload can be removed from the TUI register window.  This is done by
moving a bit more logic from show_registers into update_register_data.
After this, update_register_data simply updates the internal state,
and rerender will write to the screen.  All the actual rendering work
is done, ultimately, by display_registers_from.  This split between
updating the model and rendering makes it clear that the recursive
case can't happen any longer.

Tested-By: Tom de Vries <tdevries@suse.de>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>