radv/gfx10: make use the output usage mask when exporting NGG GS params

It shouldn't matter much because output varyings should have been
compacted during NIR shader linking but it mirrors what the driver
does when emitting NGG GS vertex parameters.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
22 jobs for !1889 with gfx10_ngg_fixes in 2 minutes and 55 seconds (queued for 3 minutes and 30 seconds)
merge request