MAINTAINERS: Move dma-buf to drm-misc git

Sumit still takes care about dma-buf, but we've merged the trees
together since way too much overlap. And Gustavo is also part of the
drm-misc team to be able to help out.

Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161116140819.31165-2-daniel.vetter@ffwll.ch