s3fb: maximize virtual vertical size for fast scrolling

Maximize virtual vertical framebuffer size during init to allow fast scrolling
(accelerated by panning).

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>