Simplify display power behavior

This is supposed to avoid too much of a hassle when enabling brightnessctl in postmarketOS to save and restore display brightness over boots.

With these settings the display just switches off after 2 mins on battery or 5 mins with power plug connected. Further settings like reducing the backlight after a while, display blanking and display sleeping are disabled, as they may cause issues when handling the backlight. Users can nonetheless enable those features in the power settings if desired.