Sync scaling DPI from Plasma to SDDM

Summary: This patch adds scaling to the list of Plasma settings that can be synced with SDDM.

Test Plan:
Displays->Scale Display->set to 1.5
Login Screen(SDDM)->Advanced->pressed "Sync"
SDDM was big.

Reviewers: #plasma, davidedmundson, ngraham

Reviewed By: #plasma, davidedmundson, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22700