sisfb: move XGI POST DDR2 bootup code into subroutines

Move DDR2 register setting code into separate subroutines. No changes
in functionality.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>