Do you use mainline kernel not the 4.4 kernel?
If so then these are the LibreELEC CEC mainline kernel changes.
You can basically fork it to your Github account and in the Debian build files change the kernel source to use yours instead. Or you can save all the individual kernel commits as patches and then include it in your Debian build. Or you can make all the kernel changes on your PC and then upload everything to a kernel folder on your Github.
I have only built LibreELEC and Armbian before so not sure how to build Debian or if it works the same way. Maybe if I have more time, I’ll try to check how it works.
Have you tried another Linux OS with a mainline kernel like Manjaro and checked if CEC works there?
Maybe it will be a good idea to check other OS’s first and find one where it works then you can look at it’s kernel source for any HDMI CEC changes that might help.