sisfb: replace outSISIDXREG with SiS_SetReg

Replace outSISIDXREG() with SiS_SetReg().

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