radv/meta: reduce vertex buffer usage in clear shaders

radv/meta: reduce vertex buffer usage in clear shaders

For depth clears we have to pass the depth in the 2nd
component, we can use push constants for some of this
later to drop the vertex buffer completely

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