This is what most seem to be using to make it work. Are you using grub and running the grub update command after adding the parameters to the grub command line? Or are you using systemd-boot which is different and requires you to add them to /etc/kernel/cmdline and then run sudo reinstall-kernels
i915.enable_dpcd_backlight=1 nvidia.NVreg_EnableBacklightHandler=0 nvidia.NVreg_RegistryDwords=EnableBrightnessControl=0