s3fb: enable DTPC

Enable Data Transfer Position Control (DTPC). This is needed at least on
Virge/DX to correctly display at higher pixclocks.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Acked-by: Ondrej Zajicek <santiago@crfreenet.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>