drm/nouveau/volt: use clamp() in nvkm_volt_map()

When it needs to get a value within a certain interval, using clamp()
makes the code easier to understand than min(max()).

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240831012803.3950100-4-lizetao1@huawei.com