MAINTAINERS: Add maintainer entry for the Nouveau DRM driver

Ben has been the maintainer of the driver even before it got included in
the kernel.

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461279842-28695-11-git-send-email-emil.l.velikov@gmail.com