st/nir: count num_uniforms for FS bultin shader

Usually the uniforms will be assigned locations and have their slots
counted automatically, but for builtin shaders the location assignment
is manual.  So count them too otherwise we get num_uniforms == 0.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
26 jobs for !341 with review/st-nir-builtin-num-uniforms in 30 minutes and 30 seconds (queued for 2 seconds)
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build:make-gallium-drivers-other

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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-drivers-other

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build:meson-gallium-radeonsi

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

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