Now that the 3 drivers using this are cleaned up we can also remove
this final bit of confusion of leaking driver internals into the
backlight power interface.

The backlight power interface itself is still a massive mess.
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>