drm/i915: Restore brightness level in aux backlight driver

Some panel will default to zero brightness when turning the
panel off and on again. This patch restores last brightness
level back when panel is turning back on.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511230225.142870-8-puthik@chromium.org