[PATCH] drm/panel: rpi-touchscreen: Add backlight support

[kbuild test robot]

Hi Nicolas,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[also build test ERROR on v4.20-rc7 next-20181218]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Nicolas-Saenz-Julienne/drm-panel-rpi-touchscreen-Add-backlight-support/20181215-164117
config: x86_64-randconfig-s3-12190437 (attached as .config)
compiler: gcc-7 (Debian 7.3.0-1) 7.3.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

All errors (new ones prefixed by >>):

>> ERROR: "devm_backlight_device_register" [drivers/gpu/drm/panel/panel-raspberrypi-touchscreen.ko] undefined!

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0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation
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