perf timechart: Add p_state_end helper

perf timechart: Add p_state_end helper

Wrap repeated code in helper functions p_state_end, which alloc a new
power_event recording last pstate, and insert to the head of
tchart->power_events.

Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220908021141.27134-5-shangxiaojing@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>