iris: Face should be a system value.

This patch adds PIPE_CAP_TGSI_FS_FACE_IS_INTEGER_SYSVAL which
despite its name is not a TGSI-specific capability, just lets
the state tracker know that it should generate a system value
for FACE.

This is needed if we want to run tgsi_to_nir on iris.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
25 jobs for !388 with iris-face in 189 minutes and 59 seconds (queued for 88 minutes and 58 seconds)
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Status Job ID Name Coverage
  Containers Build
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containers:ubuntu

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  Build+Test
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build:make-gallium-drivers-other

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build:make-gallium-drivers-radeonsi

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build:make-gallium-drivers-swr

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build:make-gallium-st-clover-llvm-39

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build:make-gallium-st-clover-llvm-4

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build:make-gallium-st-clover-llvm-5

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build:make-gallium-st-clover-llvm-6

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build:make-gallium-st-clover-llvm-7

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build:make-gallium-st-other

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build:make-loader-classic-dri

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build:make-vulkan

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build:meson-gallium-clover-llvm5

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build:meson-gallium-clover-llvm6

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build:meson-gallium-clover-llvm7

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build:meson-gallium-drivers-other

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build:meson-gallium-radeonsi

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build:meson-gallium-st-other

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build:meson-gallium-swr

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build:meson-glvnd

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build:meson-loader-classic-dri

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build:meson-vulkan

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build:scons-llvm

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build:scons-nollvm

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build:scons-swr

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