drm/i915: Don't try to initialize sprite planes on pre-ilk

We don't currently implement support for sprite planes on pre-ilk
platforms, so let's leave num_sprites at 0 so that we don't get
spurious errors during driver init.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477411083-19255-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>