How to quickly access display settings on Android Marshmallow and Lollipop phones
To create a shortcut for your Display settings, you first have to long press anywhere on your home screen, then tap on Widgets. Next, scroll down until you see the Settings widget, long press on it, and drop it on the home screen. You'll then be prompted to choose the type of widget shortcut that you're creating from a menu - just look for Display and tap on it. And that'll be all - you'll now have a Display settings shortcut at your fingertips, easily accessible at any time.
P.S.: This tutorial was made on a Nexus 5X running stock Android 6.0.1 Marshmallow. Depending on your Android version and skin, things may look different on your device.