Restore cursors across multiple screens

Summary:
4238218b762996b41480f8d1c9a1c88930e08a11 accidentally removed factoring
in m_globalPos.

BUG: 385003

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7970