Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: i386_defconfig i386):

arch/x86/kernel/cpu/mtrr/cyrix.c:99:6: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lkml.kernel.org/r/20190805201712.GA19927@embeddedor