video: fbdev: arcfb: mark expected switch fall-through

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

Addresses-Coverity-ID: 115017 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Jaya Kumar <jayalk@intworks.biz>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>