directfb: Access the screen layer through QWindow::screen()

directfb: Access the screen layer through QWindow::screen()

Instead of assuming which layer was assigned to Qt, resolve the to
be used layer via QWindow::screen()->handle(). Add a method to the
DirectFB QPlatformScreen to provide a pointer to the IDirectFBDisplayLayer

Change-Id: Iaea9466ca84daff752a4932deafbe38f48123715
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>