pwm-backlight: Disable backlight on shutdown

When a device is shut down, make sure to disable the backlight. If it
stays lit, it gives the impression that the device hasn't turned off.
Furthermore keeping the backlight on may consume power, which is not
what users expect when they shut down a device.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>