Fix stencil data unpacking in dEQP-VK.fragment_operations test

Fix stencil data unpacking in dEQP-VK.fragment_operations test

The test copies only one aspect (depth or stencil, not both) of the image
to the buffer, so host unpacking code should not use combinded DS format
 - instead S8_UIN format should be used in stencil tests.

The problem occurs in stencil tests, on implementations not supporing
S8_UINT format, where D+S format is used.

Components: Vulkan

VK-GL-CTS Issue: 3160

Affects: dEQP-VK.fragment_operations.*stencil

Change-Id: I27b69c02ebf08fa19f512cdd786ebe596d91cfeb