drm/i915/vlv: use parallel context restore when coming out of RC6

Setting this bit restores all ring contexts in parallel rather than
serially.  Matches current BWG recommendations.

Tested-by: "Meng, Mengmeng" <mengmeng.meng@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Deepak S <deepak.s@inel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>