Fix fb_ioctl dereference of null fb_info
Rethinking of how we release and free objects.
Teardown path now treats usb and framebuffer teardown more independently.
The driver can now enter a "virtual" state when usb disconnect happens
where the virtual framebuffer can live long enough (at least) to allow
clients to release cleanly.
This also lays the groundwork for hotplug reconnection of a USB display
with a particular serial # to a framebuffer that already exists for it.