backlight: Update cached value when getting the brightness

External events can cause the backlight level to change (AC adapter
plug/unplug for example) so cache the value there too.

PR: 257796
Sponsored by: Beckhoff Automation GmbH & Co. KG
MFC after: 1 week

(cherry picked from commit e26ef41f79902991c772b59927c721aa7fa5fc64)