daemon/controllers: Reintroduce a mutex around ddca_open_display2()
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)
(cherry picked from commit 7e1130ff)