dma-buf: use new iterator in dma_resv_copy_fences

This makes the function much simpler since the complex
retry logic is now handled else where.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-5-christian.koenig@amd.com