i965/fs: Refactor fs_visitor::nir_setup_inputs().

i965/fs: Refactor fs_visitor::nir_setup_inputs().

No functional change.  In preparation for supporting vertex shaders,
this adds a switch statement on shader stage (since vertex attributes
and fragment shader varyings will need different handling).  It also
renames "varying" to "input", to be more general.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>