drm/i915/bdw: Enable Logical Ring Contexts (hence, Execlists)

The time has come, the Walrus said, to talk of many things.

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