Set xrdb XFT.DPI value as integer

Summary:
QXcbScreen when loading uses QByteArray.toInt().

QByteArray("152.23").toInt() returns 0, rather than a rounded version,
this means QXcbScreen falls back to the virtualDPI for the logicalDPI
rather than the explicitly set one.

The end result is that my scaling setting didn't work properly.

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: hetzenecker, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5487