drm/mst: Some style improvements in drm_dp_mst_topology_mgr_set_mst()

Removing this lose code block and removing unnecessary bracket.

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200117015837.402239-2-jose.souza@intel.com