[platforms/drm] Do EglGbmBackend chore

Summary:
This is a small overhaul of the EglGbmBackend. Functions are restructured,
white space added and variables renamed for better readability and style
aligned with Frameworks Coding Style.

Test Plan: Session starts.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25830