backends/drm: add support for an HDR brightness setting

With this setting, the user can dim the whole screen down to a minimum brightness of 25 nits sdr brightness. Once I've written the backend in powerdevil, the brightness slider will work for HDR displays - and can be extended later to have KWin control brightness for displays in SDR mode as well.

Depends on libraries/plasma-wayland-protocols!76 (merged), and can be tested with libkscreen!185 (merged)