samsung-laptop: move code into init/exit functions

Create _init()/_exit() function for each subsystem, remove
the local struct samsung_laptop * and only keep a
struct platform_device * that can only be used in samsung_init()
and samsung_exit().

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Matthew Garrett <mjg@redhat.com>