radeonsi: use a single descriptor for the GSVS ring

radeonsi: use a single descriptor for the GSVS ring

We can hardcode all of the fields for swizzling in the geometry shader.

The advantage is that we use fewer descriptor slots and we no longer have to
update any of the (ring) descriptors when the geometry shader changes.

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