drm/i915/bdw: Write the tail pointer, LRC style

Each logical ring context has the tail pointer in the context object,
so update it before submission.

v2: New namespace.

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>