drm/radeon/kms: HDMI support for R600 KMS

Adding basic HDMI support for R600 KMS, ported from radeonhd ddx.

[airlied:- checkpatch cleanups]
Signed-off-by: Christian Koenig <deathsimple@vodafone.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>