screenpool: don't race with orderwatcher

depending on who gets signalled first the internal members of the
watcher may present an earlier state of screen setups. force a refresh
before pulling data out of the order watcher.

this resolves two problems I'm consistently experience when
disconnecting my only enabled main screen:
- the shellcorona fails to match up desktops with screens in
screenInvariants
- the screenpool's screenInvariants fails assertions when its screen
list races with the one inside orderwatcher


(cherry picked from commit 2ba7b02c)