drm/i915: Extract context backing object allocation

This is preparatory work for Execlists: we plan to use it later to
allocate our own context objects (since Logical Ring Contexts do
not have the same kind of backing objects).

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>