[5/5] d3d9: Use D3DFOG_* constants instead of plain integers

[Stefan Dösinger]

> I think this change is ok, but note that the ok() line for this test
> will still just print the numeric value.
Yes, I'm aware of that. I mainly wrote the change after I incorrectly
assumed that D3DFOG_LINEAR == 1 and got pretty confused. Writing a debug
function for better ok() lines might be a good idea too