perf core: Factor out __perf_sw_event_sched

In some cases, we need to check more than whether the software event
is enabled.  So split the condition check and the actual event
handling.  This is a preparation for the next change.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210210083327.22726-1-namhyung@kernel.org