anv: don't skip the VUE header if we are reading gl_Layer in a fragment shader

anv: don't skip the VUE header if we are reading gl_Layer in a fragment shader

This is the same we do in the GL driver: the hardware provides gl_Layer
in the VUE header, so when the fragment shader reads it we can't skip it.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>