powercap / RAPL: handle missing MSRs

Some RAPL MSRs may not exist on some CPUs, we need to continue
the topology detection and enumerate what is available.

This patch handles the missing MSRs, then reports to the powercap
layer only the features available.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>