drm/dp-mst: fix warning on unused var

Fixes: 83fa9842 ("drm/dp-mst: Drop connection_mutex check")
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011010907.103309-8-lucas.demarchi@intel.com