glsl/mesa: stop duplicating tes layout values

glsl/mesa: stop duplicating tes layout values

We already store this in gl_shader and gl_program here we
remove it from gl_shader_program and just use the values
from gl_shader.

This will allow us to keep the shader cache restore code as
simple as it can be while making it somewhat clearer where these
values originate from.

V2: remove unnecessary NULL check

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Iago Toral <itoral@igalia.com>