st/mesa: tell u_vbuf that GL core doesn't have user VBOs
st/mesa: tell u_vbuf that GL core doesn't have user VBOs

I think this only affects radeonsi - VI, because all other drivers using
u_vbuf probably don't support GL_DOUBLE, so they won't be affected by this.

Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler