drm/i915: Disable semaphore wait event idle message on BDW

According to BSpec we need to always set this magic bit in ring buffer
mode.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>