drm/amd/display: Use hardware sequencer functions for PG control

[Why & How]
These can differ per ASIC or not be present. Don't call the dcn20 ones
directly but rather the ones defined by the ASIC init table.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Eric Yang <eric.yang2@amd.com>
Acked-by: Anson Jacob <anson.jacob@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>