drm/nouveau: Ignore broken legacy I2C entries.

The nv05 card in the bug report [1] doesn't have usable I2C port
register offsets (they're all filled with zeros). Ignore them and use
the defaults.

[1] http://bugs.launchpad.net/bugs/569505

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>