drm/amd/display: make dcn20 stream_gating use a pointer for dsc_pg_control

This allows us to reuse these on different asics.

Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>