drm/i915: move flushing list processing to i915_retire_commands

... instead of threading flush_domains through the execbuf code to
i915_add_request.

With this change 2 small cleanups are possible (likewise the majority
of the patch):

- The flush_domains parameter of i915_add_request is always 0. Drop it
  and the corresponding logic.
- Ditto for the seqno param of i915_gem_process_flushing_list.

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