drm/radeon/kms: fix i2c map for rv250/280

commit 6dd66633 upstream.

Those chips have crt2_ddc bus.

Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=39672



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>