drm/amdgpu: fix memory leak in amdgpu_vm_update_page_directory

If amdgpu_ib_get() fails we returned the error code but we missed
freeing ib.

Cc: "Christian König" <christian.koenig@amd.com>
Cc: Jammy Zhou <Jammy.Zhou@amd.com>
Cc: Chunming Zhou <david1.zhou@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "monk.liu" <monk.liu@amd.com>
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org