Sometimes (like when flushing in preparation of batchbuffer execution)
we know that we'll emit a request but haven't yet done so. Allow this
case by simply taking the next seqno by default. Ensure that a request
is eventually emitted before waiting for an request by issuing it
in i915_wait_request iff this is not yet done.

Also replace one open-coded version of i915_gem_object_wait_rendering,
to prevent future code-diversion.

Chris Wilson asked me to explain and clarify what this patch does and why.
Here it goes:

Old way of moving objects onto the active list and associating them with a
reques:

1. i915_add_request + store the returned seqno somewhere
2. i915_gem_object_move_to_active (with the stored seqno as parameter)

For the current users, this is all fine. But I'd like to associate objects
(and fence regs) with the batchbuffer request deep down in the execbuf
call-chain. I thought about three ways of implementing this.

a) Don't care, just emit request when we need a new seqno. When heavily
pipelining fence reg changes, this would have caused tons of superflous
request (and corresponding irqs).

b) Thread all changed fences, objects, whatever through the execbuf-maze,
so that when we emit a request, we can store the new seqno at all the right
places.

c) Kill that seqno-threading-around business by simply storing the next
seqno, i.e. allow 2. to be done before 1. in the above sequence.

I've decided to implement c) (in this patch). The following patches are
just fall-out that resulted from this small conceptual change.

* We can handle the flushing list processing where we actually emit a flush
  (i915_gem_flush and i915_retire_commands) instead of in i915_add_request.
  The code makes IMHO more sense this way (and i915_add_request looses the
  flush_domains parameter, obviously).

* We can avoid emitting unnecessary requests. IMHO there's no point in
  emitting more than one request per batchbuffer (with or without an
  corresponding irq).

* By enforcing 2. before 1. ordering in the above sequence the seqno
  argument of i915_gem_object_move_to_active is redundant and can be
  dropped.

v2: Now i915_wait_request issues request if it is not yet emitted.
Also introduce i915_gem_next_request_seqno(dev) just in case we ever
need to do some prep work before using a new seqno.
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[ickle: Keep i915_gem_object_set_to_display_plane() uninterruptible.]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>