drm/nouveau/i2c: handle bit-banging ourselves

i2c-algo-bit doesn't actually work very well on one card I have access to
(NVS 300), random single-bit errors occur most of the time - what we're
doing now is closer to what xf86i2c.c does.

The original plan was to figure out why i2c-algo-bit fails on the NVS 300,
and fix it.  However, while investigating I discovered i2c-algo-bit calls
cond_resched(), which makes it a bad idea for us to be using as we execute
VBIOS scripts from a tasklet, and there may very well be i2c transfers as
a result.

So, since I already wrote this code in userspace to track down the NVS 300
bug, and it's not really much code - lets use it.

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