drm/i915: pipe and planes should be disabled on haswell_crtc_mode_set

So WARN in case they're not. It also does not make any sense to
wait_for_vblank at this point.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>