dma-buf: use the new iterator in dma_buf_debug_show

Simplifying the code a bit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-9-christian.koenig@amd.com