drm/i915: Move the special case wait-request handling to its one caller

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-19-git-send-email-chris@chris-wilson.co.uk