drm/amdgpu: lock both VM and BO in amdgpu_gem_object_open

drm/amdgpu: lock both VM and BO in amdgpu_gem_object_open

The VM was not locked in the past since we initially only cleared the
linked list element and not added it to any VM state.

But this has changed quite some time ago, we just never realized this
problem because the VM state lock was masking it.

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