layers: Use right index to get right descriptor

The original fix for this was commit b8fd9cc, which was reverted by
mistake in commit f45e45f.

This brings back the fix again.
diff --git a/layers/descriptor_sets.cpp b/layers/descriptor_sets.cpp
index dd1e359..efad43a 100644
--- a/layers/descriptor_sets.cpp
+++ b/layers/descriptor_sets.cpp
@@ -2208,7 +2208,7 @@
                     return false;
                 }
                 if (device_data_->device_extensions.vk_khr_sampler_ycbcr_conversion) {
-                    ImageSamplerDescriptor *desc = (ImageSamplerDescriptor *)descriptors_[index].get();
+                    ImageSamplerDescriptor *desc = (ImageSamplerDescriptor *)descriptors_[index + di].get();
                     if (desc->IsImmutableSampler()) {
                         auto sampler_state = device_data_->GetSamplerState(desc->GetSampler());
                         auto iv_state = device_data_->GetImageViewState(image_view);