s3fb: fix 15/16bpp modes with over 115MHz pixclocks on 86C365 Trio3D

Enable pixel multiplexing in 15/16bpp modes when pixclock is over 115MHz
on Trio3D (86C365) cards to fix artifacts on the left side of screen.

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>