drm/radeon/kms: fix i2c pad masks on rs4xx

These got lost in the last i2c cleanup.  Fixes:
https://bugzilla.kernel.org/show_bug.cgi?id=23222



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