panel: Expand the backlight support

At present the panel can be turned on but not off, and the brightness
cannot be controlled at run-time. Add a new API function to both the panel
and backlight uclasses to handle this. Enhance the PWM backlight driver
to deal with custom levels properly and allow the backlight to be turned
on and off.

Update the test to cover thes new features.

Signed-off-by: Simon Glass <sjg@chromium.org>