perf/core: Add the is_event_in_freq_mode() helper to simplify the code

Add a helper to check if an event is in freq mode to improve readability.

No functional changes.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250516182853.2610284-2-kan.liang@linux.intel.com