i965: Eliminate brw_nir_lower_{inputs,outputs,io} functions.

i965: Eliminate brw_nir_lower_{inputs,outputs,io} functions.

Now that each stage is directly calling brw_nir_lower_io(), and we have
per-stage helper functions, it makes sense to just call the relevant one
directly, rather than going through multiple switch statements.

This also eliminates stupid function parameters, such as the two that
only apply to vertex attributes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>