UPSTREAM: coresight: perf: Don't set the truncated flag in snapshot mode

This patch avoids setting the truncated flag when operating in snapshot
mode since the trace buffer is expected to be truncated and discontinuous
from one snapshot to another.  Moreover when the truncated flag is set
the perf core stops enabling the event, waiting for user space to consume
the data.  In snapshot mode this is clearly not what we want since it
results in stale data.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit 5aafd9bf7aa932f2a97e5a55d1acda67c161621f).

Bug: 140266694
Change-Id: If37e9ad3e246f1e2b869726078bacaf3ae85fd91
Signed-off-by: Yabin Cui <yabinc@google.com>