radv: disable vertex reuse when writing viewport index

This fixes some issues we'd hit later if using viewport
indexes.

Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/amd/vulkan/radv_cmd_buffer.c

@@ -511,6 +511,8 @@ radv_emit_vertex_shader(struct radv_cmd_buffer *cmd_buffer,
 			       cull_dist_mask << 8 |
 			       clip_dist_mask);
 
+	radeon_set_context_reg(cmd_buffer->cs, R_028AB4_VGT_REUSE_OFF,
+			       S_028AB4_REUSE_OFF(vs->info.vs.writes_viewport_index));
 }