perf parse-events: Use get/put_events_file()

Instead of accessing the trace_events_path variable directly, that may
not have been properly initialized wrt detecting where tracefs is
mounted.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-id7hzn1ydgkxbumeve5wapqz@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>