drm/dp_mst: Fix indenting in drm_dp_mst_topology_mgr_set_mst()

This has always bugged me but somehow I've never remembered to actually
fix it. So let's do that.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117224749.128994-1-lyude@redhat.com