drm/amdgpu: fix amdgpu_mn_unlock() in the CS error path

Avoid unlocking a lock we never locked.

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