drivers/perf: hisi_pcie: Merge find_related_event() and get_event_idx()

The function xxx_find_related_event() scan all working events to find
related events. During this process, we also can find the idle counters.
If not found related events, return the first idle counter to simplify
the code.

Signed-off-by: Junhao He <hejunhao3@huawei.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20240223103359.18669-8-yangyicong@huawei.com


Signed-off-by: Will Deacon <will@kernel.org>