drm/nouveau/bar: swap oneinit/init ordering, and rename bar3 to bar2

NVIDIA call it BAR2, Linux APIs treat it as BAR3 due to BAR1 being a
64-bit BAR, which I presume take two slots or something.

No actual code changes here, just to make future commits less messy.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>