drm/i915: put runtime PM only at the end of intel_mark_idle

Because intel_mark_idle still touches some registers: it needs the
machine to be awake. If you set both the autosuspend and PC8 delays to
zero, you can get a "Device suspended" WARN when gen6_rps_idle touches
registers.

This is not easy to reproduce, but happens once in a while when
running pm_pc8.

Testcase: igt/pm_pc8
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -8215,10 +8215,8 @@ void intel_mark_idle(struct drm_device *dev)
 
 	dev_priv->mm.busy = false;
 
-	hsw_package_c8_gpu_idle(dev_priv);
-
 	if (!i915.powersave)
-		return;
+		goto out;
 
 	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
 		if (!crtc->fb)
@@ -8229,6 +8227,9 @@ void intel_mark_idle(struct drm_device *dev)
 
 	if (INTEL_INFO(dev)->gen >= 6)
 		gen6_rps_idle(dev->dev_private);
+
+out:
+	hsw_package_c8_gpu_idle(dev_priv);
 }
 
 void intel_mark_fb_busy(struct drm_i915_gem_object *obj,