v3d: rename vertex shader key (num)_fs_inputs fields

v3d: rename vertex shader key (num)_fs_inputs fields

Until now this made sense because we always paired vertex shaders
with fragment shaders, but as soon as we implement geometry and
tessellation shaders that will no longer be the case, so rename
this to (num_)used_outputs.

v2: Use 'used_outputs' instead of ns_outputs, which is more explicit (Eric).

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Eric Anholt <eric@anholt.net>