panel-samsung-s6e3ha2.c:undefined reference to `backlight_device_unregister'

[kbuild test robot]

Hi Hoegeun,

FYI, the error/warning still remains.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   ea094f3c830a67f252677aacba5d04ebcf55c4d9
commit: ed29f9426d9bf1b5ec7e79dfb28f39de7b3a0069 drm/panel: Add support for S6E3HA2 panel driver on TM2 board
date:   8 weeks ago
config: i386-randconfig-x0-06041640 (attached as .config)
compiler: gcc-6 (Debian 6.2.0-3) 6.2.0 20160901
reproduce:
        git checkout ed29f9426d9bf1b5ec7e79dfb28f39de7b3a0069
        # save the attached .config to linux build tree
        make ARCH=i386 

All errors (new ones prefixed by >>):

   drivers/built-in.o: In function `s6e3ha2_remove':
>> panel-samsung-s6e3ha2.c:(.text+0x156ba0): undefined reference to `backlight_device_unregister'
   drivers/built-in.o: In function `s6e3ha2_probe':
>> panel-samsung-s6e3ha2.c:(.text+0x15768b): undefined reference to `backlight_device_register'
   panel-samsung-s6e3ha2.c:(.text+0x157728): undefined reference to `backlight_device_unregister'

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