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.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
(cherry picked from commit 1c92358b)