MAINTAINERS: add maintainers for DRM STM driver

Add Philippe Cornu and myself as maintainers.

Signed-off-by: Yannick Fertre <yannick.fertre@st.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1492164819-10513-6-git-send-email-yannick.fertre@st.com


Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Eric Anholt <eric@anholt.net>