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

MAINTAINERS

@@ -4502,6 +4502,8 @@ F:	Documentation/devicetree/bindings/display/st,stih4xx.txt
 DRM DRIVERS FOR STM
 M:	Yannick Fertre <yannick.fertre@st.com>
 M:	Philippe Cornu <philippe.cornu@st.com>
+M:	Benjamin Gaignard <benjamin.gaignard@linaro.org>
+M:	Vincent Abriou <vincent.abriou@st.com>
 L:	dri-devel@lists.freedesktop.org
 T:	git git://anongit.freedesktop.org/drm/drm-misc
 S:	Maintained