wm: call displayReady for all built in displays when system is ready.

When system has more than one built in display, displayReady needs
to be called for all built in displays when window manager is ready.
Otherwise, some system services, such as presentation, mediarouter,
etc, won't work on these displays.

Bug: 24103683
Change-Id: Ibf08074eff555c14a318236bd06e7b4855503140