drm/radeon: fix dpms on/off on trinity/aruba v2

The external encoder need to be setup again before enabling the
transmiter. This seems to be only needed on some trinity/aruba
to fix dpms on.

v2: Add comment, only setup again on dce6 ie aruba or newer.

Cc: <stable@kernel.org>
Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>