drm: panel/simple: add backlight dependency

The simple panel code uses the backlight interface to
find a device, which fails when backlight is disabled:

drivers/built-in.o: In function `panel_simple_platform_probe':
:(.text+0xd3c48): undefined reference to `of_find_backlight_by_node'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@gmail.com>