drm/radeon/kms: set i2c adapter class to I2C_CLASS_DDC

The most common use of the radeon i2c buses is for ddc.

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