backlight: Remove unneeded owner field

Remove uneeded owner field from backlight_properties structure.

Nothing uses it and it is unlikely that it will ever be used. The
backlight class uses other means to ensure that nothing references
unloaded code.

Based on a patch from Dmitry Torokhov <dtor@insightbb.com>

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>