radv: fix vkUpdateDescriptorSets with inline uniform blocks

descriptorCount is the number of bytes into the descriptor, so
it shouldn't be used as an index. srcArrayElement/dstArrayElement
specify the starting byte offset within the binding to copy from/to.

This fixes new CTS tests:
dEQP-VK.binding_model.descriptor_copy.*.inline_uniform_block_*
dEQP-VK.binding_model.descriptor_copy.*.mix_3
dEQP-VK.binding_model.descriptor_copy.*.mix_array1

Fixes: 8d2654a4 ("radv: Support VK_EXT_inline_uniform_block.")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 7562a2cb)