drm/radeon/kms: rework combios i2c handling

Handle asic specific table to hw mappings in
combios_setup_i2c_bus() directly.  This allows us
to remove most of the combios quirks and clean up
the i2c bus setup.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>