isl/formats: R8G8B8_UNORM_SRGB isn't supported on HSW

Merged Faith Ekstrand requested to merge gfxstrand/mesa:review/isl-rgb-snorm-no-hsw into master

On Haswell, the format works but it doesn't properly do an sRGB decode. It appears to act identically to R8G8B8_UNORM. Only Vulkan uses this format so this only affects Vulkan on HSW.

Cc: mesa-stable@lists.freedesktop.org

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