sisfb: replace inSISIDXREG with SiS_GetReg

Replace inSISIDXREG() with SiS_GetReg().
Signed-off-by: NAaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>