Let dma_fence::signaled, etc, read directly from the address that the hw
is writing with updated completed fence seqno, so we can potentially
notice that the fence is signaled sooner.

Plus add some docs.
Signed-off-by: NRob Clark <robdclark@chromium.org>
Link: https://lore.kernel.org/r/20210726144359.2179302-2-robdclark@gmail.comSigned-off-by: NRob Clark <robdclark@chromium.org>