EDID parsing: strings are up to 13 bytes

Summary:
The descriptor blocks with strings come every 18 bytes,
5 are the header, e.g. for the model:
[0x00 0x00 0x00 0xFC 0x00] followed by the data (13 bytes).

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14410