perf tools: Use evsel->attr.sample_type instead of session->sample_type

Eventually session->sample_type will go away as we want to support
multiple sample types per session, so use it from the evsel which is a
step in that direction.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-0vwdpjcwbjezw459lw5n3ew1@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>