cocoa: Suppress Cocoa window with -display

Do not open a Cocoa window when another display is selected that will be
initialized later. The Cocoa display cannot be selected with -display,
so there is no need to check its argument.

Signed-off-by: Rainer Müller <raimue@codingfarm.de>
Reviewed-by: Andreas Färber <andreas.faerber@web.de>
Message-id: 1441807710-25431-1-git-send-email-raimue@codingfarm.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
