video: fbdev: omapfb_main: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: omap1_defconfig arm):

drivers/watchdog/wdt285.c:170:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/watchdog/ar7_wdt.c:237:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:449:23: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1549:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1547:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1545:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1543:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1540:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1538:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/video/fbdev/omap/omapfb_main.c:1535:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>