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)

Co-authored-by: Jakob Petsovits <jpetso@petsovits.com>