drm/vc4: Use runtime autosuspend to avoid thrashing V3D power state.

The pm_runtime_put() we were using immediately released power on the
device, which meant that we were generally turning the device off and
on once per frame.  In many profiles I've looked at, that added up to
about 1% of CPU time, but this could get worse in the case of frequent
rendering and readback (as may happen in X rendering).  By keeping the
device on until we've been idle for a couple of frames, we drop the
overhead of runtime PM down to sub-.1%.

Signed-off-by: Eric Anholt <eric@anholt.net>