Comment 6 for bug 1518473

link-dupont really nailed it! I am having the same issue on an HP Elitebook 8770w with ONLY an NVIDIA Quadro K3000M, using the proprietary driver version 387.34 on Fedora 27, Kernel 4.14.3-300

I tried all:

acpi_backlight=video
acpi_backlight=vendor
acpi_backlight=native

to no avail, so removed all of that and installed inotify-tools, I do have /sys/class/backlight/acpi_video0 so I ended up with the following script in /home/YOURUSERHERE/Scripts/backlight_inotify.sh

#!/bin/sh

path=/sys/class/backlight/acpi_video0

luminance() {
    read -r level < "$path"/actual_brightness
    factor=$((100 / max))
    printf '%d\n' "$((level * factor))"
}

read -r max < "$path"/max_brightness

nvidia-settings -n -a BacklightBrightness="$(luminance)"

inotifywait -me modify --format '' "$path"/actual_brightness | while read; do
    nvidia-settings -n -a BacklightBrightness="$(luminance)"
done

And then created file brightnessfix.desktop under /home/YOURUSERHERE/.config/autostart

[Desktop Entry]
Name=Brightness Fix
Exec=/home/YOURUSERHERE/Scripts/backlight_inotify.sh
Terminal=false
Type=Application
Name[en_US]=backlightfix.desktop

It is a very effective workaround, thanks.