I have a different Clevo model (P751DM-G) but with same issue: the
backlight Fn-keys doesn't work neither out of the box, nor with a kernel
4.6. Well, in fact, they work until I suspend/resume, and then they no
longer work. Looking at acpi_listen, I noticed that after first
suspend/resume cycle, some ACPI events where no longer triggered (like
the backlight Fn-keys, but also the lid close!).

I found a fix by loading a clevo-wmi driver. There are several clevo-wmi 
drivers on the internet (various attempts to write a driver for it...). For my 
laptop, I use:
* https://bitbucket.org/lynthium/clevo-xsm-wmi (it aims at supporting all Clevo 
models)

Since the clevo-xsm-wmi driver doesn't seem to handle your P65 model, you may 
need to try some other ones, like this one:
* https://github.com/gamaral/clevo-wmi
Maybe it could help for you too.

To be honest, I have no idea why it did fix my backlight fn-keys, as
those drivers are supposed to handle different features (keyboard
backlight, rfkill switch, hw monitoring). Nevertheless, if I unload the
module, then the issue happen again.

Note that this workaround fixes the Fn-keys: the backlight popup notification 
appears and I can increase/decrease value, however it has no effect on the 
screen. This is just ACPI event that has been fixed.
In order to really adjust LCD backlight, I have to play with nvidia-settings 
(with proprietary driver). Example, to set backlight brightness at 50%:

nvidia-settings -n -a BacklightBrightness=50

Some glue is still missing to link ACPI events with nvidia-settings.

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https://bugs.launchpad.net/bugs/1535470

Title:
  Backlight brightness keys not working

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