drm/radeon/kms: set encoder type to DVI for HDMI on evergreen

Fixes the pink line that shows up with some hdmi monitors.  This
will need to be revisited when audio support is added.

Fixes:
http://bugs.freedesktop.org/show_bug.cgi?id=27452

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>