[AD] Small problem with driver removals

[Chris <kcat@xxxxxxxxxx>]

Since the DGA1 driver has been removed (which the public sf.net CVS 
server still doesn't show) and the VBEAF driver has been disabled by 
default, this causes a problem when a program uses the specific GFX_* 
macros. Especially since the VBEAF driver can still be re-enabled, I 
think we need to do something about it.

What I propose is to map GFX_VBEAF (when disabled) and the old DGA GFX 
macros (which I can't remember, ATM) to 0 so old programs that reference 
them will still compile, though those drivers won't be able to be used.

- Kitty Cat