perfmon: fix pmc hw indices out-dated when multiple pmc

perfmon: fix pmc hw indices out-dated when multiple pmc

When adding two or more events using a single "set pmc",
the pmc hardware indices might be out-dated due to kernel
reschdeduling the perf_event hardware counters.
E.g. set pmc cpu-cycles cache-misses

Solution:
Open and enable all the events first, then aquire the
indices from the kernel.

Change-Id: I6913a871ab169e3b2855ac6159f527a1fca343e9
Signed-off-by: Su Wang <su.z.wang@ericsson.com>