perf_plugins_handler: Fix coordination-only value aggregation

When there's coordination only, i.o.w. plugins-handler is used
without plugins, the internal tables of aggretation values are
not updated.

Fix this by invoking plugins_policy_sync_level_limits() also for
the case of no plugins, such that the coordination/aggregation of
values can be correctly performed.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I6fb51d20ccf121fba850475c95380fce218d8a71