drm/radeon: add load detection support for ext DAC on R200 (v2)

The R200 asics use an external DAC for the secondary DAC.
The current KMS code tries to use code for the integrated
TV DAC for R200 which leads to unpredictable results since
R200 does not have an integrated TV DAC. This patch ports
the external DAC load detection support from the UMS
driver to KMS.

v2: fix typo in loop break logic

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Egbert Eich <eich@suse.de>