nir/lower_mediump_io: Don't remap base unless needed

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.

Fixes: fb29cef8dda ("nir: add many passes that lower and optimize 16-bit input/outputs and samplers")
Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11732>