i915: Don't attempt to short-circuit object_wait_rendering by checking domains.

This could return early when reading after writing a buffer, if somebody
had already put it on the flushing list (write domains are 0, but still
active), leading to glReadPixels failure.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>