isl/formats: R8G8B8_UNORM_SRGB isn't supported on HSW

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
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
(cherry picked from commit 7c1b39cf)