Add empty static inline function fallbacks if backlight support
is not enabled, so that drivers using the backlight don't fail to
build.

Fixes:
In function `simple_panel_ioctl':
undefined reference to `of_backlight_find'
undefined reference to `backlight_set_brightness_default'
undefined reference to `backlight_set_brightness'

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>