Fix resource buffers used for the Op?Convert graphics tests

Fix resource buffers used for the Op?Convert graphics tests

I didn't catch this when reviewing CL 2240. The "graphics"
variants of the OpUConvert/OpSConvert/OpFConvert tests are
writing the shader results to a UBO instead of an SSBO. UBOs
are read-only and writing to one has undefined results.

This CL simply changes the resources to use SSBOs rather than
UBOs, which follows the same template of most of the other
dEQP-VK.spirv_assembly.instruction.graphics.* tests.

Affects:

dEQP-VK.spirv_assembly.instruction.graphics.sconvert.*
dEQP-VK.spirv_assembly.instruction.graphics.uconvert.*
dEQP-VK.spirv_assembly.instruction.graphics.fconvert.*

Change-Id: Iab4014e6ca5e1482f74ff6d8bec7f04793b0e69d
Components: Vulkan