Fix screen brightness adjustment through function keys

Not all machines provide levels that scale linearly, e.g. an ThinkPad X230
has 16 levels of exponential scale that end with [50, 67, 100].

Make acpivout_find_brightness() return the BCL's index instead of its value
such that acpivout_brightness_step() is able to fall back to the lower next
value by index in case the 5% heuristic fails.

Tested by Tracey Emery with an T470s
OK patrick jcs

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