drm/i915/dp_mst: Add missing DSC compression disabling

Add the missing DSC compression disabling step for MST streams,
similarly to how this is done for SST outputs.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231030155843.2251023-18-imre.deak@intel.com