anv: Only consider minSampleShading when sampleShadingEnable is set
From the Vulkan 1.1.107 spec:
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.
In other words, if sampleShadingEnable is set to VK_FALSE, we should ignore minSampleShading.