ivi-shell: simplify ivi_layout_screen_set_render_order

It is not necessary check the ivi-id of pregiven objects (layer or
surface). Traversing the list of all objects is sure to find the exact
same pointer we start with, bugs aside.

The controller modules are responsible for providing valid pointers. We
cannot protect from invalid pointers anyway.

Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
[Pekka: fix subject, add commit message]
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>