drm/radeon/kms: unify i2c handling

Previously we added i2c buses as needed when enumerating connectors
power management, etc.  This only exposed the actual buses used and
could have lead to the same buse getting created more than once if
one buses was used for more than one purpose.  This patch sets up
all i2c buses on the card in one place and users of the buses just
point back to the one instance.

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