glsl: Add plumbing for handling uniform binding qualifiers.

glsl: Add plumbing for handling uniform binding qualifiers.

Sampler uniforms and uniform blocks do not have a var->constant_value.
Instead, they have an integer var->binding value.

This makes extending set_uniform_initializer() somewhat problematic: it
assumes that there is an ir_constant * which represents the initializer,
and that it's safe to dereference that without any NULL checks.

Instead, this patch creates an analogous function for binding
qualifiers, and calls one or the other as appropriate.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>