sna: Be a little more assertive in retiring after set-domain

sna: Be a little more assertive in retiring after set-domain

After a successful set-domain for writing with the CPU, we know that the
buffer is idle so remove it from our request tracking. (External clients
complicate matters in that they may keep the bo active even after our
set-domain.) On the contrary, because of read-read optimisations a bo
may still be active after a set-domain for reading by the CPU, in which
we need to remain conservative in retiring the bo.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>