drm/i915/dp_mst: Enable decompression in the sink from the MST encoder hooks

Enable/disable the DSC decompression in the sink/branch from the MST
encoder hooks. This prepares for an upcoming patch toggling DSC for each
stream as needed, but for now keeps the current behavior, as DSC is only
enabled for the first MST stream.

v2:
- Rebased on latest drm-tip.

Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107001505.3370108-5-imre.deak@intel.com