perf session: Move ui_progress_update in __perf_session__process_events()

The progress bar is changed when the file offset changes. This happens only
when the next mmap is done. No need to call ui_progress_update() for every
event.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <20101130163820.094836523@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>