MAINTAINERS: update drm/stm maintainers list

Add Benjamin Gaignard and Vincent Abriou as STM maintainers:
Benjamin Gaignard <benjamin.gaignard@linaro.org>
Vincent Abriou <vincent.abriou@st.com>

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1495544446-22360-1-git-send-email-vincent.abriou@st.com