drm/i915: Extract the actual workload submission mechanism from execbuffer

So that we isolate the legacy ringbuffer submission mechanism, which becomes
a good candidate to be abstracted away. This is prep-work for Execlists (which
will its own workload submission mechanism).

No functional changes.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>