winemac: Check the display link running state as well as the window count to decide whether to start/stop it.

The display link may be stopped even if there are associated windows, due to
idleness.  So, it may need to be started when a window is added even if it's
not the first window.
Signed-off-by: Ken Thomases <ken@codeweavers.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>