i965/shader: Pull setup_image_uniform_values out of backend_shader

i965/shader: Pull setup_image_uniform_values out of backend_shader

I tried to do this once before but Curro pointed out that having it in
backend_shader meant it could use the setup_vec4_uniform_values helper
which did different things in vec4 and fs.  Now the setup_uniform_values
function differs only by an assert in the two backends so there's no real
good reason to be using it anymore.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>