UPSTREAM: perf/arm-cmn: Streamline node iteration

Refactor the places where we scan through the set of nodes to switch
from explicit array indexing to pointer-based iteration. This leads to
slightly simpler object code, but also makes the source less dense and
more pleasant for further development. It also unearths an almost-bug
in arm_cmn_event_init() where we've been depending on the "array index"
of NULL relative to cmn->dns being a sufficiently large number, yuck.

Bug: 247832918
Change-Id: I4aa4971d8fd00e70ab30391445d3e006b2e47e40
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/ee0c9eda9a643f46001ac43aadf3f0b1fd5660dd.1638530442.git.robin.murphy@arm.com


Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit da5f7d2c)
Signed-off-by: Robin Peng <robinpeng@google.com>