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

[kbuild test robot]

Hi Dave,

It's probably a bug fix that unveils the link errors.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   32c1431eea4881a6b17bd7c639315010aeefa452
commit: ab6eb211b07a42a6346e284056422fd9a8576a99 Merge tag 'drm/panel/for-4.12-rc1' of git://anongit.freedesktop.org/tegra/linux into drm-next
date:   9 weeks ago
config: x86_64-randconfig-h0-06120933 (attached as .config)
compiler: gcc-4.9 (Debian 4.9.4-2) 4.9.4
reproduce:
        git checkout ab6eb211b07a42a6346e284056422fd9a8576a99
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

   drivers/built-in.o: In function `s6e3ha2_remove':
>> panel-samsung-s6e3ha2.c:(.text+0x1a238e): undefined reference to `backlight_device_unregister'
   drivers/built-in.o: In function `s6e3ha2_probe':
>> panel-samsung-s6e3ha2.c:(.text+0x1a257d): undefined reference to `backlight_device_register'
   panel-samsung-s6e3ha2.c:(.text+0x1a2672): undefined reference to `backlight_device_unregister'

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