intel/blorp: Use an actual chunk of vertex buffer for the VUE header

intel/blorp: Use an actual chunk of vertex buffer for the VUE header

We're about to start passing other things in as a sort of "VS header" for
vertex shaders and we need a place to put them.  Since we want the instance
id to be one of them, it makes sense to have one vec4 that's either VUE
header or VS header.  Always uploading some handy zeros makes the code a
bit simpler.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>