drm/i915/bdw: Implement WaForceContextSaveRestoreNonCoherent

v2: Reorder defines (Ben)
v3: More bikesheds, this time re-ordering comments! (Chris)

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Resolve conflict.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>