drm/i915: Drop bogus fbdev sprite disable code

plane->enabled is never set, so this code didn't do anything.

Also drm_fb_helper_restore_fbdev_mode() will now disable all cursors
and sprites for us, so we don't have to bother anymore.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>