dri-devel mailing list moved - update MAINTAINERS

I posted to dri-devel@lists.sourceforge.net, and got a bounce back:

   The dri-devel list has moved to freedesktop.org (see
   http://lists.freedesktop.org/mailman/listinfo/dri-devel

). If you were
   subscribed to the list here, the subscription should have been
   transferred to the new location.

   Please only post to the new list.

Fix MAINTAINERS to correspond.

Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>