perf: Optimize __pmu_ctx_sched_out()

There is is no point in doing the perf_pmu_disable() dance just to do
nothing. This happens for ctx_sched_out(.type = EVENT_TIME) for
instance.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240807115550.392851915@infradead.org