dma-buf: use new iterator in dma_resv_test_signaled

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

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005113742.1101-8-christian.koenig@amd.com