drm/i915: Extract ringbuffer destroy & generalize alloc to take a ringbuf

More prep work: with Execlists, we are going to start creating a lot
of extra ringbuffers soon, so these functions are handy.

No functional changes.

v2: rename allocate/destroy_ring_buffer to alloc/destroy_ringbuffer_obj
because the name is more meaningful and to mirror a similar function in
the context world: i915_gem_alloc_context_obj(). Change suggested by Brad
Volkin.

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>