Int 10 Fn 12  - Video - Alternate Function Select (ps) - Display-switch I  [V]

   AH = 12h
   BL = 35h
   AL = subfunction
       00h initial adapter video off
       01h initial planar video on
       02h switch active video off
       03h switch inactive video on
       80h *UNDOCUMENTED* set system board video active flag
   ES:DX -> buffer for 128 byte save area (if AL = 00h-03h)

Return: AL = 12h if function supported
Desc:  switch between two video adapters which may otherwise have address
     conflicts