drm/amdkfd: use kvcalloc() instead of kvmalloc() in kfd_migrate

simplify programming with existing functions.

Signed-off-by: Yang Wang <KevinYang.Wang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>