drm: panel/sharp: add backlight dependency

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

drivers/built-in.o: In function `sharp_panel_probe':
:(.text+0x5ceac): undefined reference to `of_find_backlight_by_node'

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