i965/fs: Rework uniform handling

i965/fs: Rework uniform handling

Previously, we treated the entire UNIFORM file as if it had two elements:
One for direct things and one for indirect.  This is substantially
different from how the old visitor code handled it where each element was
effectively its own uniform.  This commit makes the NIR path more like the
old ir_visitor path where each uniform is separate.  This should allow us
to more easily make decisions about what to push.

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