drm/amdgpu: fix potential resource leak warning

Clear resource leak warning that when the prepare fails,
the allocated amdgpu job object will never be released.

Signed-off-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>