video: fbdev: sis: fix duplicated code for different branches

Refactor code in order to avoid identical code for different branches.

This issue was detected with the help of Coccinelle.

Addresses-Coverity-ID: 1226761
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>