drm/radeon/kms: i2c s/sprintf/snprintf/g for safety

As per advice from Jean Delvare.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>