Perf: arm64: tracectr: initialize counts after hotplug

Due to the mismatch between counter values saved across CPU hotplug
by perf and tracecounter logic, incorrect values are calculated
for a cpu's first entry in trace buffer.

Ignore the first entry when CPU comes out of hotplug and set previous
counter values in this scenario.

Change-Id: I0a61943d14aa3bc96b50f7ed05e25c592be9aa35
Signed-off-by: Neil Leeder <nleeder@codeaurora.org>