nir/lower_mediump_io: Don't remap base unless needed
Otherwise drivers that don't use 16-bit slots for varyings will get confused and have their driver_locations scribbled over. This has caused multiple problems for both Panfrost and Asahi this week. Given the only other user of the pass for varyings is radeonsi, which needs both together, I think this is the least controversial fix.
Cc @jekstrand @mareko