drm/radeon/kms: fix up r1xx-rs4xx i2c buses

Fix the vbios mapping and only add the actual
buses that the cards have.  The existing code was
mostly correct.  Just clean up a few cases on r2xx/r3xx
and document that buses the hw actually has.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>