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

drivers/gpu/drm/display/drm_dp_mst_topology_internal.h

@@ -10,7 +10,9 @@
 #ifndef _DRM_DP_MST_HELPER_INTERNAL_H_
 #define _DRM_DP_MST_HELPER_INTERNAL_H_
 
-#include <drm/display/drm_dp_mst_helper.h>
+struct drm_dp_sideband_msg_req_body;
+struct drm_dp_sideband_msg_tx;
+struct drm_printer;
 
 void
 drm_dp_encode_sideband_req(const struct drm_dp_sideband_msg_req_body *req,