Properly enable Sample Shading in Fragment Shading Rate tests

Properly enable Sample Shading in Fragment Shading Rate tests

Vulkan spec states:

> Sample shading is enabled for a graphics pipeline:
>
> * If the interface of the fragment shader entry point of the graphics
>   pipeline includes an input variable decorated with SampleId or
>   SamplePosition. In this case minSampleShadingFactor takes the value
>   1.0.
> * Else if the sampleShadingEnable member of the
>   VkPipelineMultisampleStateCreateInfo structure specified when
>   creating the graphics pipeline is set to VK_TRUE. In this case
>   minSampleShadingFactor takes the value of
>   VkPipelineMultisampleStateCreateInfo::minSampleShading.
>
> Otherwise, sample shading is considered disabled.

This change ensures that sample shading is properly enabled in the
Fragment Shading Rate tests relying on it by using gl_SampleId.

Components: Vulkan

Affects:
dEQP-VK.fragment_shading_rate.sampleshadinginput.*

VK-GL-CTS Issue: 2966

Change-Id: I7fad0ed7694a2d26ef7db9ea268fa5d6f93264a6