[platforms/drm] Refactor out Edid class

Summary:
The new class is responsible for parsing EDID blobs. It has pretty
straightforward API: you hand blob data to the constructor and after
that you're ready to query parsed data, e.g. physical size, etc.

The main reason to extract EDID parsing code into a class is to clean up
drm_output.cpp a bit.

Test Plan: Compiles, the DRM platform still works.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D22211