drm/amdgpu: move lockdep assert to the right place.

Since newly added BOs don't have any mappings it's ok to add them
without holding the VM lock. Only when we add per VM BOs the lock is
mandatory.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Bhardwaj, Rajneesh <Rajneesh.Bhardwaj@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>