videomode: provide dummy inline functions for !CONFIG_OF

If CONFIG_OF is disabled but we build any driver that tries to
use of_get_display_timing, we get this link error:

ERROR: "of_get_display_timing" [drivers/gpu/drm/rcar-du/rcar-du-drm.ko] undefined!

This adds an empty inline function as an alternative, to avoid changing
each driver using this function when we want to build them without
CONFIG_OF.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>