xwayland: Remove unnecessary xwl_window_is_toplevel() check from xwl_output_set_window_randr_emu_props()

Since the recent fix to call xwl_output_set_window_randr_emu_props() from
ensure_surface_for_window(), it is now only called on a toplevel window,
so the is-toplevel check is not necessary for the
xwl_output_set_window_randr_emu_props() case.

This commit moves the check to xwl_output_set_randr_emu_prop_callback()
so that we only do it when we are walking over all Windows of a client
to update the property on a change of the emulated resolution.

Acked-by: Olivier Fourdan <ofourdan@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>