perf/arm-cmn: Fix filter_sel lookup

Carefully considering the bounds of an array is all well and good,
until you forget that that array also contains a NULL sentinel at
the end and dereference it. So close...

Reported-by: Qian Cai <quic_qiancai@quicinc.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/bebba768156aa3c0757140457bdd0fec10819388.1652217788.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>