drm/i915: Fix RC6 suspend/resume

We restored RC6 twice on resume, even with modesetting off.  Instead,
only restore it once and skip RC6 initialization entirely in non-KMS mode.

Signed-off-by: Andy Lutomirski <luto@mit.edu>
Tested-by: Jeff Chua <jeff.chua.linux@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>