Fix the following fallthrough warning (arm64-randconfig with Clang):

drivers/video/fbdev/xilinxfb.c:244:2: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60edca25.k00ut905IFBjPyt5%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>