Int 10 Fn 12  - Video - Alternate Function Select (vga, Mcga) - Gray-scal  [V]

   AH = 12h
   BL = 33h
   AL = new state
       00h enable gray scale summing
       01h disable gray scale summing

Return: AL = 12h if function supported
Desc:  specify whether or not colors should be converted to gray scale when
     palette or color registers are loaded

See Also: AX=101Bh,AX=BF06h,AX=BF08h