waylandserver: only signal lockScreenShown once it has actually been shown

When the system goes to suspend and screen locking for suspend is enabled,
suspend is inhibited until ScreenLocker::KSldApp::self()->lockScreenShown()
gets called, in order to make sure that the lockscreen is shown before the
system goes to standby, and thus also when the system wakes (instead of
potentially sensitive user information).
However, signalling that when the lockscreen gets mapped can't work reliably,
as it's then a matter of timing whether or not KWin actually presents an
image with the lockscreen before suspending. To fix that, this commit replaces
that logic with only calling lockScreenShown once every output actually got a
lockscreen presented.