drm: Remove the dithering_mode_property field

Unfortunately, I haven't been thorough enough in:

  commit ddecb10c
  Author: Lespiau, Damien <damien.lespiau@intel.com>
  Date:   Tue Aug 20 00:53:04 2013 +0100

      drm: Remove drm_mode_create_dithering_property()

And forgot to remove the dithering_mode_property member of struct
drm_mode_config.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>