radv: fix out-of-bounds access when copying descriptors BO list

We shouldn't increment the buffer list pointers twice.

This fixes some crashes with new CTS
dEQP-VK.binding_model.descriptor_copy.*.

Cc: 18.3 19.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 9256e0a0)