drm/i915: Don't update the render-clock for every bo.

Only update the render-clock on transition from busy to idle and vice
versa, or else we burn a significant percentage of the cpu just rewriting
the register -- not quite as power-friendly as intended ;-)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -4052,8 +4052,13 @@ void intel_mark_busy(struct drm_device *dev, struct drm_gem_object *obj)
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return;
 
+	if (!dev_priv->busy) {
 		dev_priv->busy = true;
 		intel_increase_renderclock(dev, true);
+	} else {
+		mod_timer(&dev_priv->idle_timer, jiffies +
+			  msecs_to_jiffies(GPU_IDLE_TIMEOUT));
+	}
 
 	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
 		if (!crtc->fb)