drm/nouveau/i2c: tidy up bit-bang helpers, also fixing nv50 setsda bug

Was using nv_mask, which is bad.  Reading the reg senses the current line
states, which aren't necessarily the states we're trying to drive the
lines to.

Fixed to store SCL driver state just as we already do for SDA.

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