perf pmu: Skip invalid hybrid pmu

On hybrid platform, such as Alderlake, if atom CPUs are offlined,
the kernel still exports the sysfs path '/sys/devices/cpu_atom/' for
'cpu_atom' pmu but the file '/sys/devices/cpu_atom/cpus' is empty,
which indicates this is an invalid pmu.

Need to check and skip the invalid hybrid pmu.

Before:

  # perf list
  ...
  branch-instructions OR cpu_atom/branch-instructions/ [Kernel PMU event]
  branch-instructions OR cpu_core/branch-instructions/ [Kernel PMU event]
  branch-misses OR cpu_atom/branch-misses/           [Kernel PMU event]
  branch-misses OR cpu_core/branch-misses/           [Kernel PMU event]
  bus-cycles OR cpu_atom/bus-cycles/                 [Kernel PMU event]
  bus-cycles OR cpu_core/bus-cycles/                 [Kernel PMU event]
  ...

The cpu_atom events are still displayed even if atom CPUs are offlined.

After:

  # perf list
  ...
  branch-instructions OR cpu_core/branch-instructions/ [Kernel PMU event]
  branch-misses OR cpu_core/branch-misses/           [Kernel PMU event]
  bus-cycles OR cpu_core/bus-cycles/                 [Kernel PMU event]
  ...

Now only cpu_core events are displayed.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20210708013701.20347-2-yao.jin@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>