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