powercap: intel_rapl: Use index to initialize primitive information

Currently, the RAPL primitive information array is required to be
initialized in the order of enum rapl_primitives.
This can break easily, especially when different RAPL Interfaces may
support different sets of primitives.

Convert the code to initialize the primitive information using array
index explicitly.

No functional change.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Tested-by: Wang Wendy <wendy.wang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>