drm/amdgpu: fix comment on amdgpu_bo_va

Except for the reference count all other members are protected
by the VM PD being reserved.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>