drm/amdgpu: fix error handling in amdgpu_vm_init

When clearing the root PD fails we need to properly release it again.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org