dma-buf: stop using the dmabuf->lock so much v2

The attachment list is now protected by the dma_resv object.

Stop holding the dma_buf->lock while calling ->attach/->detach,
this allows for concurrent attach/detach operations.

v2: cleanup commit message and locking in _debug_show()

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/336790