X11: fix kded xcb resource leak

This fixes commit 579358f5

XOpenDisplay() internally calls xcb_connect(), creating a new XCB client
connection. This means that on every KScreen configuration change event
the kded5 process creates a new connection until XCB runs into the hard
limit of 256 of clients, leading to the dreaded "Maximum number of
clients reached" error.

Re-use the display from QX11Info instead of calling XOpenDisplay().

BUG: 453280


(cherry picked from commit 0270a493)