[QNX] Enable MT screen event handling on Blackberry

[QNX] Enable MT screen event handling on Blackberry

Consuming incoming screen events through the BPS event
dispatcher on its current incarnation has proven to be
very inefficient since it cannot put up with the screen
event throughput. This patch enables the screen event
thread also for Q_OS_BLACKBERRY platforms, in addition
to stock QNX. This behavior can still be configured
through the project file, though.

Task-number: QTBUG-26177
Change-Id: I98a3b22549c9fbf0c16b5a8c39a55c1eaa6ec1f4
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>