backlight: Add backlight handle to display to sync blanking

When the display is blanked, the backlight should be switched off two. So I added the missing backlight = property to the display.

This fixes also the '-@ problem', where the backlight was not switched on when the devicetree was compiled wiht symbols. The reason was simple: the driver assumes that when the bl node has a phandle, there will be someone holding this handle to enable and disable the backlight. The -@/--symbols option adds a phandle to every not for alter usage, so the drivers guess is wrong.

BCS 746-000121