Leaking driver internal tracking into the already massively confusing
backlight power tracking is really confusing.

Stop that by allocating a tiny driver private data structure instead.
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>