drm/i915: Nuke ORIGIN_GTT

There is no users of it, so no need to keep handling for it.

Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210815014346.373945-2-jose.souza@intel.com