drm/dp_mst: Fix W=1 warnings

Fix the warnings that show up with W=1.
They are all about unused but set variables.
If functions returns are not used anymore make them void.
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200205084842.5642-1-benjamin.gaignard@st.com