Simplify getting edid in xrandr backend

Summary: XRandR::outputEdid is used in only one place, let it return a byte array directly.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: zzag, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D14044