x86/events: Mark expected switch-case fall-throughs

In preparation to enable -Wimplicit-fallthrough by default, mark
switch-case statements where fall-through is intentional, explicitly in
order to fix a couple of -Wimplicit-fallthrough warnings.

Warning level 3 was used: -Wimplicit-fallthrough=3.

 [ bp: Massasge and trim commit message. ]

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jacek Tomaka <jacek.tomaka@poczta.fm>
Cc: Jia Zhang <qianyue.zj@alibaba-inc.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190125184917.GA7289@embeddedor