perf: Optimize context reschedule for single PMU cases

Currently re-scheduling a context will reschedule all active PMUs for
that context, even if it is known only a single event is added.

Namhyung reported that changing this to only reschedule the affected
PMU when possible provides significant performance gains under certain
conditions.

Therefore, allow partial context reschedules for a specific PMU, that
of the event modified.

While the patch looks somewhat noisy, it mostly just propagates a new
@pmu argument through the callchain and modifies the epc loop to only
pick the 'epc->pmu == @pmu' case.

Reported-by: Namhyung Kim <namhyung@kernel.org>
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/20240807115549.920950699@infradead.org