[x86 setup] The current display page is returned in %bh, not %bl

The current display page is an 8-bit number, even though struct
screen_info gives it a 16-bit number.  The number is returned in %bh,
so it needs to be >> 8 before storing.

Special thanks to Jeff Chua for detailed bug reporting.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>