[effects/blur] Disable texture cache on Wayland
It is known to create problems especially on multi-screen and is not as
important on Wayland as it used to be on X11 thanks to things like
buffer age and in general better rendering.
Test Plan: Opened the config module and verified that the option is
Subscribers: plasma-devel, kwin
Differential Revision: https://phabricator.kde.org/D9879