Change backlight to not always be on

-Since our backlight driver on the display debug board has a shared
enable/PWM pin, enable-gpios property results in backlight always on.
Modify so that backlight_enable in startek driver turns on PWM
-Add failure messages to backlight to be used in rack testing

Change-Id: Ic0177914a0294f53325d078b01125ce72a72092a