drm/i915: Don't try to use SPR_SCALE when we don't have a sprite scaler

Haswell does not have a scaler in the sprite pipeline anymore, so let's
ensure:
  1/ We bail out of update_plate() when someone is trying to ask to
     display a scaled framebuffer,
  2/ We never write to the nonexistent SPR_SCALE register

v2: Smash in the fixup from Damien in the disable_plane function.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com> (for v1)
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> (for v1)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>