drm/i915: Fix flushing regression from 9af90d19f

Whilst moving the code around in 9af90d19f, I dropped the or'ing in of
new write domains which would zero out the write domain for a render
target if later reused as a source later in the batch. This meant that
we might drop a required flush before reading from the render target.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31043
Reported-by: xunx.fang@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>