[media] imon: clean up disconnect routine

- Eliminate a possible circular locking lockdep warning
- Make sure we don't try to unregister a vfd on a device w/a vga screen
- Always free imon context after devices are removed (display_close can
  just error out w/no context)

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>