radv: Prepare for not using the guard band for lines & points.

radv: Prepare for not using the guard band for lines & points.

Vulkan Clipping is defined in terms of vertices, the scissor based
clipping happens on pixels. There is a difference with points and
lines, as a vertex can be outside the viewport while some pixels are in.
On Vulkan thoise pixels shouldn't be drawn, while they would be with
the guardband.

Signed-off-by: Bas Nieuwenhuizen <basni@google.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>