drm/amd/pm: Use kmemdup instead of kmalloc and memcpy

Fixes coccicheck warning:
drivers/gpu/drm/amd/pm/powerplay/hwmgr/vega12_processpptables.c:255:
36-43: WARNING opportunity for kmemdup

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>