Fix undersized buffer use in extend operands tests

Fix undersized buffer use in extend operands tests

Prior to this patch, the dEQP.image.extend_operands_spirv1p4.* tests
used the size of the input image as the buffer size. Since the tests
reuse the buffer to read back the output image (which is potentially
much larger, due to the unconditional use of R32G32B32A32_*INT formats),
this resulted in corruption of device memory.

Affects: dEQP-VK.image.extend_operands_spirv1p4.*

Components: Vulkan

VK-GL-CTS issue: 1980

Change-Id: I8d07cd7d54cc305635b9674074a7eafa52bf6dfa
(cherry picked from commit b38bedda7faf25272716ee234812d432637972e3)