i965: Use nir_lower_tex for texture coordinate lowering

i965: Use nir_lower_tex for texture coordinate lowering

Previously, we had a rescale_texcoords helper in the FS backend for
handling rescaling of texture coordinates.  Now that we can do variants in
NIR, we can use nir_lower_tex to do the rescaling for us.  This allows us
to delete the i965-specific code and gives us proper TEXTURE_RECTANGLE and
GL_CLAMP handling in vertex and geometry shaders.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>