Set initial count of screens before calling Screens::init

Screens::init sets up a Qt::QueuedConnection connect from countChanged
to changed. This means that when first setting up the connection a
changed signal will be delivered in the next event cycle. This can mess
with other code. E.g. WaylandServer recreating outputs which can make
Xwayland crash or KWin internally crash.

To solve this problem we just need to make sure to set the output count
to the initial value before performing the init.

Reviewed-By: bshah