radv/meta: add common shader vertex generation function

radv/meta: add common shader vertex generation function

Instead of passing in the same 1.0, -1.0 combinations via
vertex buffers, we can just use vertex id to have the vertex
shader build them. This function introduces the generator
code needed, later patches will use this.

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