drm/nouveau: do a better job at hiding the NIH i2c bit-banging algo

I'd like to export the corresponding functions from the i2c core
so that I can use them in fallback bit-banging in i915.ko

v2: Adapt to new i2c export patch.

Cc: nouveau@lists.freedesktop.org
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>