perf/x86: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword as a replacement for
a /* fall through */ comment, instead of letting the code fall through
to the next case.

Notice that Clang doesn't recognize /* fall through */ comments as
implicit fall-through markings.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://github.com/KSPP/linux/issues/115