drm/i915/bxt: Add WaForceContextSaveRestoreNonCoherent

Note that we also need this for skl.

Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Note that we also need this for skl, requested by Imre.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>