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: NDaniel Vetter <daniel.vetter@intel.com>
Acked-by: NDaniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>