drm/amd/display: Write DSC enable to MST DPCD

Rework the dm_helpers_write_dsc_enable callback to
handle the MST case.

Use the cached dsc_aux field.

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Signed-off-by: David Francis <David.Francis@amd.com>
Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>