perf/arm-smmuv3: Fix lockdep assert in ->event_init()

perf/arm-smmuv3: Fix lockdep assert in ->event_init()

Same as
https://lore.kernel.org/all/20240514180050.182454-1-namhyung@kernel.org/,
we should skip `for_each_sibling_event()` for group leader since it
doesn't have the ctx yet.

Fixes: f3c0eba28704 ("perf: Add a few assertions")
Reported-by: Greg Thelen <gthelen@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: Tuan Phan <tuanphan@os.amperecomputing.com>
Signed-off-by: Chun-Tse Shao <ctshao@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241108050806.3730811-1-ctshao@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>