radeonsi: Fix handling of geometry shader output vertex ID

radeonsi: Fix handling of geometry shader output vertex ID

It needs to increment at shader runtime, not at shader compile time, as
the geometry shader can emit vertices in loops. LLVM automagically
converts the ID back to an immediate value if its value can be
determined at compile time.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>