video: omapfb: Fix fall-through warning for Clang

Fix the following fallthrough warnings:

drivers/video/fbdev/omap/omapfb_main.c:1558:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
           case 0:
           ^
   drivers/video/fbdev/omap/omapfb_main.c:1558:2: note: insert 'break;' to avoid fall-through
           case 0:
           ^
           break;
   1 warning generated.

This helps with the ongoing efforts to globally enable
-Wimplicit-fallthrough for Clang.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/lkml/202110141005.hUjaYMEi-lkp@intel.com/


Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211014165320.GA1145571@embeddedor