Consider rounding mode differences in mipmap tests

Consider rounding mode differences in mipmap tests

The tests already tolerate a threshold for rounding mode differences
for the result of a downscaling vkCmdBlitImage, however the accumulated
difference between Round-To-Nearest and Round-To-Zero may exceed the
current threshold. With this change, the expected result is generated
from the preceding device-rendered mipmap instead, so any legitimate
error will always be within the threshold now.

Affects:

dEQP-VK.api.copy_and_blit.core.blit_image.all_formats.generate_mipmaps.*_linear

Components: Vulkan

VK-GL-CTS issue: 1106

Change-Id: I0999419320d1b2384f42fbc8eddda29e79147bd4
(cherry picked from commit b861ca1e6d4c002d4359a71dbff1cdf898e79aca)