drm/i915: Refactor the wait_rendering completion into a common routine

Harmonise the completion logic between the non-blocking and normal
wait_rendering paths, and move that logic into a common function.

In the process, we note that the last_write_seqno is by definition the
earlier of the two read/write seqnos and so all successful waits will
have passed the last_write_seqno. Therefore we can unconditionally clear
the write seqno and its domains in the completion logic.

v2: Add the missing ring parameter, because sometimes it is good to have
things compile.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>