radv: move radv_declare_shader_args() out of shader_variant_compile()

Declaring them earlier will allow us to access them in NIR.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timur Kristóf <timur.kristof@gmail.com>
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