drm/amd/powerplay: fix memdup.cocci warnings

Simplify the code a bit by using kmemdup instead of kzalloc and memcpy.

Generated by: scripts/coccinelle/api/memdup.cocci

CC: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Likun Gao <Likun.Gao@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>