radv: don't copy buffer descriptors list for samplers

Sampler descriptors don't have a buffer list.

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

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 4924dfc8)