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>

drivers/gpu/drm/i915/intel_display.c

@@ -4593,7 +4593,7 @@ void intel_init_clock_gating(struct drm_device *dev)
 	 * GPU can automatically power down the render unit if given a page
 	 * to save state.
 	 */
-	if (I915_HAS_RC6(dev)) {
+	if (I915_HAS_RC6(dev) && drm_core_check_feature(dev, DRIVER_MODESET)) {
 		struct drm_gem_object *pwrctx;
 		struct drm_i915_gem_object *obj_priv;
 		int ret;