drm/i915: fix WaDisablePSDDualDispatchEnable on VLV v2

Can prevent a hang when we get to tessellation.  We need to set bit 15
as well for this workaround.

v2: update changelog with accurate info

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>