power: use systemd-logind's new SetBrightness() D-Bus API to set the display brightness
systemd 243 should gain support to set the display brightness (commit 2a66c2). From the NEWS
:
systemd-logind now exposes a per-session
SetBrightness()
bus call, which may be used to securely change the brightness of a kernel brightness device, if it belongs to the session's seat. By using this call unprivileged clients can make changes to "backlight" and "leds" devices securely with strict requirements on session membership. Desktop environments may use this to generically make brightness changes to such devices without shipping private SUID binaries or specific udev rules for that purpose.