sddm-theme: Use non-deprecated KDEInstallDirs variable

Summary:
Also drop the prefix and use the KDE_INSTALL_FULL.. variant for an
absolute path (similar to logo=${KDE_INSTALL_FULL_DATADIR}...).

Test Plan:
Checked that the path of the background image is the same as
before.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27241