graphics: trivial optimization of old-style graphic mode

graphics: trivial optimization of old-style graphic mode

Trivially optimize the old-style graphics by using another level of
temporary buffering in bitplane format, thereby enabling the use of
rep movsd to write the contents to the screen.  This can speed things
up substantially in virtualization environments.

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