[kstyle] Update blur region even if Breeze has no X11 support

Summary:
KWindowEffects provides a platform-independent API for enabling blur,
which requires neither Xlib nor XCB.

Test Plan: Compiles.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26631