Honor animation brightness threshold also for sysfs helper

We should only animate the screen brightness if there are enough steps available to make for a smooth transition. This check was forgotten for the new sysfs brightness animation.

BUG: 426883 FIXED-IN: 5.20.0

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Animation unchanged on my machine, changed

To simulate added the following into ~/.config/powerdevilrc:

[Brightness]
brightnessAnimationThreshold=5000

Brightness wasnt animated anymore

The default threshold is 100, so should fix the bug reporter's machine where there are only 15 setps.

I'd like a respin for 5.20 powerdevil, please, sorry.

@ngraham @davidedmundson @jriddell