MAINTAINERS: Add dri-devel as a mailing list for anything fbdev

fbdev is in maintenance only, except that it's still used by drm
through the drm fbdev emulation, to be able to use fbcon. And people
might want to sometimes extend fbcon to enable new features for drm
drivers, e.g. Hans' panel orientation work.

The problem is that when those patches only touch fbdev code they'll
never show up on drm developer's radar, which means we end up with
designs that don't really fit whell into the full stack. That happened
a bit with the panel orientation work, where an fbcon patch made it
into 4.14, implementing a design that won't really work on the drm
side. Which means we now have to redo things, and on top coordinate 2
subsystem trees.

Since fbdev is super low-volume we can prevent this in the future by
simply adding the dri-devel mailing list to the fbdev subsystem.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Acked-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170908153528.17528-1-daniel.vetter@ffwll.ch