video: fbdev: sis_main: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 115025
Addresses-Coverity-ID: 115026
Addresses-Coverity-ID: 115027
Addresses-Coverity-ID: 115028
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>