drm/i915:Initialize the second BSD ring on BDW GT3 machine

Based on the hardware spec, the BDW GT3 machine has two independent
BSD ring that can be used to dispatch the video commands.
So just initialize it.

V3->V4: Follow Imre's comment to do some minor updates. For example:
more comments are added to describe the semaphore between ring.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
[danvet: Fix up checkpatch error.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>