drm/i915: Wait for vblank in hsw_enable_ips()

Now that the vblank wait is gone from intel_enable_primary_plane(),
hsw_enable_ips() needs to do the vblank wait itself.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>