sisfb: move the CONFIG warning to sis_main.c

sis_main.c is always compiled, so we can check Kconfig options there.

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