MAINTAINERS: Maarten for drm-misc co-maintainer

I'm stepping down, also handing all the drm-misc stuff to the new
team. Plan is that Sean handles 4.17, and Maarten then has fun with
4.18 as his first release.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180131102156.25634-1-daniel.vetter@ffwll.ch