nir/linker: disable varying from uniform lowering by default

nir/linker: disable varying from uniform lowering by default

This fixes performance regression for Specviewperf/Energy
on AMD GPU. Other GPUs passing varying by memory may choose
to re-enable it as need.

Fixes: 26046250437 ("nir/linker: support uniform when optimizing varying")
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15341>