drm/amdgpu: fix memory leak during TDR test(v2)

fix system memory leak

v2:
fix coding style

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>