perf report: Correct display of samples and events in header

This patch prints the number of samples and the count of performance
events separately.

This allows comparing performance of different applications with each
other.

Previously, the sample count was displayed against an 'Events:' heading.
With this patch, the header now reads (for example):

Samples: 5K of event 'instructions'
Event count (approx.): 2993026545

The patch covers both the stdio and the browser interface.

Signed-off-by: Ashay Rane <ashay.rane@tacc.utexas.edu>
[ committer note: Fixed wrt e7f01d1 ]
Link: http://lkml.kernel.org/n/tip-h4nfjm8msedlk8gxkzivfh5y@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>