radv: Don't use a virtual function for getting the buffer virtual address.

radv: Don't use a virtual function for getting the buffer virtual address.

We are really not going to use a winsys which does not need to store
the va, so might as well store it in a standard field.

Not sure this helps perf much though, as most of the cost is in the
cache miss accessing the bo anyway, which we stil need to do.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>