drm/radeon/kms: enable HDMI audio on RS600/RS690/RS740

We will need method of selecting encoder that should receive HDMI block. For
now we assign HDMI block to first enabled encoder. Hopefully there are not many
RS6x0 chips with two digital encoders.

[airlied: add RS740 checks as per Alex suggestion.]

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