Add support for fixed time brightness fading

Basically we have 4 brightness levels (off, lpm, dim, on). Normally all of these have different values, so getting a fade request to the same target level == caused by the same (display state) transition and we should no reprogram the timings as that will cause hiccups. But the intent was that it is allowed to cut *down* the duration (i.e. if slower als tuning somehow manages to take place before faster state transition fading and both have the same target level). Need to check if this gets blocked now.