drm/radeon/kms: clean HDMI definitions

We already know same offsets are used for different encoders/transmitters, so
just numeric them instead naming incorrectly. Additionaly we found additional
registers needed for RV770+

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>