radv: always emit a position export in gs copy shaders

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Fixes: f8d03372 ('radv: add multiple streams support for the GS copy shader')
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
38 jobs for !1895 with radv_gs_fixes in 15 minutes and 45 seconds (queued for 3 seconds)
merge request