perf/x86/intel: Mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

arch/x86/events/intel/core.c: In function ‘intel_pmu_init’:
arch/x86/events/intel/core.c:4959:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
   pmem = true;
   ~~~~~^~~~~~
arch/x86/events/intel/core.c:4960:2: note: here
  case INTEL_FAM6_SKYLAKE_MOBILE:
  ^~~~
arch/x86/events/intel/core.c:5008:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
   pmem = true;
   ~~~~~^~~~~~
arch/x86/events/intel/core.c:5009:2: note: here
  case INTEL_FAM6_ICELAKE_MOBILE:
  ^~~~

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

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>