drm/i915: Remove 'retire' parameter from intel_fb_obj_flush

Setting retire=true is identical to using origin=ORIGIN_CS, so make the
same simplification to intel_fb_obj_flush() as already employed for
intel_fb_obj_invalidate().

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/20170222114049.28456-6-chris@chris-wilson.co.uk