drm: unconditionally clean up dma buffers of closing clients

With the last patch to ditch DMA_QUEUE support, we should be able
to call the dma cleanup uncoditionally, even when the master has
disappeared.

Do so because it just makes more sense.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>