drm/i915/bdw: GEN-specific logical ring set/get seqno

No mistery here: the seqno is still retrieved from the engine's
HW status page (the one in the default context. For the moment,
I see no reason to worry about other context's HWS page).

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>