drm/radeon: add a missing entry to encoder_names

An entry for INTERNAL_VCE encoder was missing. Add it.

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>