Return the xcb_screen_t and not the QPlatformScreen

from QXcbNativeInterface. The QPlatformScreen is available from
QScreen::handle()

Change-Id: If81daf34c07f4a49c85c43d3755d1a9167626d6d
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>