radeonsi: fix an off-by-one error in the bounds check for max_vertices

radeonsi: fix an off-by-one error in the bounds check for max_vertices

The spec actually says that calling EmitStreamVertex is undefined when
you exceed max_vertices. But we do need to avoid trampling over memory
outside the GSVS ring.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>