dzn: Fix 3D to 2D image copies
The unintentional CTS uprev in !26736 (merged) found one fail among the deadlocks (which are a WARP bug and will be addressed via a new NuGet separately). Not sure why this coverage was missing before, but apparently it was. The relevant copy that was problematic has a source layer count of 1, and a dest layer count of 16. This loop should handle the larger of the layer counts.