powercap/rapl: reduce ipi calls

Reduce remote CPU calls for MSR access by combining read
modify write into one function.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>