drm/amdgpu: fix CS error handling v2

Stop double freeing the the BO list by pulling the content
of amdgpu_cs_parser_prepare_job() into the IOCTL function again.

v2: better commit message

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com> (v1)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>