drm/i915: Fix !CONFIG_PM sysfs for real this time

Somehow this hunk got dropped from my last patch. We do not have the
rc6_attrs when there is no CONFIG_PM so this causes a compilation error.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>