drm/amd/display: Free DCN version of stream encoder

Cross a TODO item off the list. Cleanup SIGNAL_TYPE_HDMI_FRL, it's
not currently supported.

Signed-off-by: Eric Bernstein <eric.bernstein@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>