[XRandRBrightness] Don't call for xrandr if it's not available

Review Request #126146 - Created Nov. 23, 2015 and submitted

Information
Kai Uwe Broulik
powerdevil
Plasma/5.5
352462
Reviewers
solid
luebking

Check whether the extension is there before calling into it.

Also cache calls to QX11Info::connection() if we're at it.

Still works, didn't test without xrandr.

Thomas Lübking
Kai Uwe Broulik
Kai Uwe Broulik
Review request changed

Status: Closed (submitted)

Change Summary:

Submitted with commit dde0ecce02259a26655737e6674950e2e303bc6f by Kai Uwe Broulik to branch Plasma/5.5.
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