drm/radeon/kms: Simplify I2C post_xfer function

There is no point in re-doing in post_xfer all the initialization
that was already done by pre_xfer. Instead, only do the work which
differs from pre_xfer.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>