drm/amd/display: add debugfs sdp hook up function for Navi

[why]
need to send immediate SDP message via debugfs on Navi board.

[how]
hook up the DCN1x encoder function of sending immediate sdp
message to DCN2.

Signed-off-by: David (Dingchen) Zhang <dingchen.zhang@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>