drm/i915: Abstract the legacy workload submission mechanism away

As suggested by Daniel Vetter. The idea, in subsequent patches, is to
provide an alternative to these vfuncs for the Execlists submission
mechanism.

v2: Splitted into two and reordered to illustrate our intentions, instead
of showing it off. Also, remove the add_request vfunc and added the
stop_ring one.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet:
- Make checkpatch happy.
- Be grumpy about the excessive vtable.
- Ditch gt->is_ring_initialized.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>