ivi-layout: simplify API

Use assert() to check for invalid NULL arguments. Something like that happens
during development and assert() makes it easier to find the error. And it avoids
unnecessary additional error handling.
The hmi-controller asserted anyways so this just moves the assert on level
deeper. Other controller probably do the same thing, or don't check the return
values at all.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>