amd/powerplay: use kmemdup instead of kmalloc + memcpy

Save a few clocks by replacing calls to kmalloc followed by memcpy
with a single call to kmemdup.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>