perf/arm-cmn: Ensure dtm_idx is big enough

perf/arm-cmn: Ensure dtm_idx is big enough

While CMN_MAX_DIMENSION was bumped to 12 for CMN-650, that only supports
up to a 10x10 mesh, so bumping dtm_idx to 256 bits at the time worked
out OK in practice. However CMN-700 did finally support up to 144 XPs,
and thus needs a worst-case 288 bits of dtm_idx for an aggregated XP
event on a maxed-out config. Oops.

Fixes: 23760a014417 ("perf/arm-cmn: Add CMN-700 support")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/e771b358526a0d7fc06efee2c3a2fdc0c9f51d44.1725296395.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>