Fix Plasma applet printer job count

Summary: We were always showing the jobs for all printers, not for the currently open printer item

Test Plan:
* Have two printers setup (ideally one that is not connected)
* Print to the non connected printer
* See how without this patch the applet says "there's 1 job" for both printers
* See that with this patch it only says "there's 1 job" for the correct printer

Reviewers: dantti

Reviewed By: dantti

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D10351