tdfxfb: do not make changes to default tdfx_fix

This was suggested by Geert Uytterhoeven to avoid overwriting of default
values from the tdfx_fix.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>