drm/dp_mst: avoid includes in drm_dp_mst_topology_internal.h

Prefer forward declarations over includes where possible.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/83d96b40724e7fd18bec81a9c6c935dbe924da91.1709898638.git.jani.nikula@intel.com