drm/nouveau/i2c: use a custom bitbanging delay for the adt7473

This patch adds a way to define a custom delay when scanning for i2c devices
because the adt7473 sometimes doesn't like the default bitbanging udelay.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>