pan/bifrost: Manually constant fold register class

Fixes errors for some people building Mesa:

../src/panfrost/bifrost/bifrost_sched.c:32:31: error: initializer
element is not constant
 const unsigned max_vec2_reg = max_primary_reg / 2;

../src/panfrost/bifrost/bifrost_sched.c:33:31: error: initializer
element is not constant
 const unsigned max_vec3_reg = max_primary_reg / 4; // XXX: Do we need
to align vec3 to vec4 boundary?

../src/panfrost/bifrost/bifrost_sched.c:34:31: error: initializer
element is not constant
 const unsigned max_vec4_reg = max_primary_reg / 4;

../src/panfrost/bifrost/bifrost_sched.c:35:32: error: initializer
element is not constant
 const unsigned max_registers = max_primary_reg +

../src/panfrost/bifrost/bifrost_sched.c:40:28: error: initializer
element is not constant
 const unsigned vec2_base = primary_base + max_primary_reg;

../src/panfrost/bifrost/bifrost_sched.c:41:28: error: initializer
element is not constant
 const unsigned vec3_base = vec2_base + max_vec2_reg;

../src/panfrost/bifrost/bifrost_sched.c:42:28: error: initializer
element is not constant
 const unsigned vec4_base = vec3_base + max_vec3_reg;

../src/panfrost/bifrost/bifrost_sched.c:43:27: error: initializer
element is not constant
 const unsigned vec4_end = vec4_base + max_vec4_reg;

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
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  Build+Test
passed meson-arm64 #508881

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passed meson-armhf #508880

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passed meson-clang #508874

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passed meson-clover #508878

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passed meson-i386 #508882

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passed meson-main #508877

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passed meson-swr-glvnd #508873

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passed meson-vulkan #508879

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passed scons-llvm #508884

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passed scons-nollvm #508883

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passed scons-swr #508875

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passed scons-win64 #508876

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  Test
passed test-llvmpipe-gles2 1/4 #508885

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passed test-llvmpipe-gles2 2/4 #508886

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passed test-llvmpipe-gles2 3/4 #508887

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passed test-llvmpipe-gles2 4/4 #508888

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