Fix buffer size in manual zeroing Workgroup aliased memory

Fix buffer size in manual zeroing Workgroup aliased memory

The buffer A needs to be large enough to fit the largest case when
using the smallest type, which is an array of uint8_t to cover an
array of 4 struct {dvec4, dvec4}.  Update the buffer size to properly
cover the entire array.

Component: Vulkan
VK-GL-CTS Issue: 2524

Affected tests:
- dEQP-VK.compute.workgroup_memory_explicit_layout.zero.*

Change-Id: I098b037314fb87fbe03769a0a4fe79edcf46143e