MAINTAINERS: Add drm-misc

KS happened, time to make this official!

Well, scripting work to make this all happen is still ongoing, but I'm
trying to at least keep the new tree semi-in-sync with the temporary
topic/drm-misc branch in the drm-intel.git repo. So for now still no
new committers, and existing ones still need to push to topic/drm-misc
in drm-intel.git.

Big kudos to Jani&Sean for volunteering as co-maintainers!

v2: Restrict patterns a bit to avoid all the driver spam.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Acked-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161116140819.31165-1-daniel.vetter@ffwll.ch