Add support for fixed time brightness fading

If blanking happens due to power key press, then we are dealing with response time - where in
the good old 0.1/1/10 second scale 100ms is still in the instant category, but ...

Previously the display power off took N ms, where N is close to zero. When fade to black is
added, it takes fade_duration-N ms more, i.e. something that used to be instantaneous now
takes longer-> someone somewhere is bound to get irritated even by that, let alone with anything
longer lasting.

Anyway, the fade-out was not supposed to happen for in-call proximity blanking -> need to fix it.