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daemon/controllers: Reintroduce a mutex around ddca_open_display2()

Jakob Petsovits requested to merge work/cherry-pick-7e1130ff into Plasma/6.1

Cherry-pick MR for !390 (merged).


ddcutil keeps some global state, and in particular we've seen a crash in ddc_close_display() related to its open_displays hashmap.

One likely culprit for such a crash is that brightness workers run on separate threads, one per DDCutilDisplay object, so they could get executed simultaneously when more than one external monitor is involved.

A less likely, but still possible culprit is when a DDCutilDisplay constructor opens a display handle at the same time that a brightness worker sets the brightness of a different display on a different thread.

We could look into combining all brightness setter calls into a single BrightnessWorker that gets shared across DDCutilDisplay objects, but this would introduce extra complexity and still wouldn't address the constructor vs. brightness setter issue.

So this commit reintroduces a mutex to isolate all of these usages from each other. We had one in 5.x, which was removed in 6.0 at my (in hindsight incorrect) request: !312 (comment 865976), !312 (comment 867942)

BUG: 489169

(cherry picked from commit 7e1130ff)

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