Prefer to set current mode in resolution combobox

Summary:
Preferred resolution may be lower than current resolution (e.g. on KVM),
in this case it will be selected in the combobox instead of current
resolution. Select current resolution if it's available.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16400