staging: vboxvideo: Use more drm_fb_helper functions

Store fbhelper and afb struct directly in vbox_private and use
drm_fb_helper_fbdev_setup to replace vbox_fbdev_init, note we cannot use
drm_fb_helper_fbdev_teardown since we use a private framebuffer for the
fbdev.

And replace vbox_driver_lastclose with drm_fb_helper_lastclose.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>