Have you installed AMD's open source graphics driver.
I have two solutions that might work. The first one is a bit harder but shouldn't produce a performance hit. Using a text editor of your choosing, open /etc/profile.d/kwin.sh then proceed to add the line: exportKWIN_TRIPLE_BUFFER=1. Check to see if that doesn't solve your issue. If not then try disabling vsync on your driver by using the commands listed on this article on the ArchWiki.
The other method is easier to do but might have a performance hit when doing graphical tasks like gaming. First go to System Settings on KDE, locate Display and Monitor then click on Compositor. Once there go to the Tearing prevention ("vysnc") option in the menu and set it to Full screen repaints.
If all goes well it should eliminate your screen tearing, either method should solve the problem.