drm/i915: Split the ringbuffers from the rings (1/3)

As advanced by the previous patch, the ringbuffers and the engine
command streamers belong in different structs. This is so because,
while they used to be tightly coupled together, the new Logical
Ring Contexts (LRC for short) have a ringbuffer each.

In legacy code, we will use the buffer* pointer inside each ring
to get to the pertaining ringbuffer (the actual switch will be
done in the next patch). In the new Execlists code, this pointer
will be NULL and we will use instead the one inside the context
instead.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>