powercap: intel_rapl: Optimize rp->domains memory allocation

In the memory allocation of rp->domains in rapl_detect_domains(), there
is an additional memory of struct rapl_domain allocated, optimize the
code here to save sizeof(struct rapl_domain) bytes of memory.

Test in Intel NUC (i5-1135G7).

Signed-off-by: xiongxin <xiongxin@kylinos.cn>
Tested-by: xiongxin <xiongxin@kylinos.cn>
Reviewed-by: Srinivas <Pandruvada&lt;srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>