drm/nv40: rework lvds table parsing

All indications seem to be that the version 0x30 table should be handled
the same way as 0x40 (as used on G80), at least for the parts that we
currently try use.

This commit cleans up the parsing to make it clearer about what we're
actually trying to achieve, and unifies the 0x30/0x40 parsing.

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