drm/i915/mst: add mst sub-struct to struct intel_dp

drm/i915/mst: add mst sub-struct to struct intel_dp

Move active_mst_links, mst_encoders[], and mst_mgr members of struct
intel_dp under an mst sub-struct to group mst related things together.

Rename them active_links, stream_encoders[] and mgr for clarity.

Note that is_mst and mst_detect are not included, as they're also
relevant for non-mst. The sub-struct is for active mst.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6f282f90bfe2dd9162e2dee8f681c84313971992.1740746939.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>