radeonsi: Refactor initialization of shader export intrinsic arguments.
radeonsi: Refactor initialization of shader export intrinsic arguments.

In preparation for extending this code, which would make it rather unwieldy in
its current place.

Signed-off-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>
Reviewed-by: Christian König <c7ea837d7a46effe4232b086213468b8b31643bf@amd.com>
Reviewed-by: Alex Deucher <08dc22c6156113f2deff178e35e3ed9b24d6af9e@amd.com>

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