QNX: Fix invalid screen size when attaching a second display

When attaching a display several SCREEN_EVENT_DISPLAY events might be received.
SCREEN_PROPERTY_SIZE might not be populated in the first event we get, so we wait.

Change-Id: I357108bb347eb92cd773d293bfe7cd462fb7e527
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Petr Nejedlý <pnejedly@blackberry.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>