fbdev: tdfxfb: Make the VGA register initialisation a bit more obvious

fbdev: tdfxfb: Make the VGA register initialisation a bit more obvious

For a while I was trying to get this working on m68k, for some reason
the card doesn't display anything if it's own video BIOS didn't run..

Anyhow, I spent a long time looking up what each of these offsets
were to work out what the code is configuring and eventually
replaced them with the human readable defines in video/vga.h.

Functionally there is no change but maybe it makes it a bit easier to
look at for the next person that finds themselves in here.

Tested on a real voodoo 3 on x86_64.

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
Signed-off-by: Helge Deller <deller@gmx.de>