The following use scenarios are now possible:

1. When no display connected, primary HwcDisplay is created in headless
   mode.

2. When user connects first display, it binds to primary slot, replacing
   headless HwcDisplay.

3. When user connects another display it binds to the new HwcDisplay
   slot, creating new display for the framework.

4. When user disconnects first (Primary) display, drm_hwc detaches
   second display and attaches it to the Primary slot. In this case
   framework is notified as Primary display resolution updated
   (Plugged->Plugged transition). And second display is disconnected
   (Plugged->Unplugged transition).

DrmDisplayPipeline is now created on demand (after hotplug event).

HwcDisplay class is now destructed on connector unplug, which will give
us ability to destroy any resource caches (will be required for FB
caching logic).

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>