drm/i915: Factor out setup_private_pat()

Factor out setup_private_pat() for introducing the following patches.

Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Ben Widawsky <benjamin.widawsky@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1505202148-22959-1-git-send-email-zhi.a.wang@intel.com


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>