XRandR: Generic code cleanup

Summary:
No functional changes, white space added where sensible, debug messages now
without repetition of qCDebug(..) macros, modernized, keeps line limit.

Test Plan: Compiles and KScreen runs on X11.

Reviewers: #kwin

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D23647