drm/exynos: added comments and code clean.

this patch adds the following comments and code clean.
- add comment of exynos_drm_crtc_apply() call at page flip time.
- add comment that when exynos_drm_fbdev_reinit() is called,
  why num_connector is 0 and also the framebuffers should be destroyed.
- remove buf_off member from struct exynos_drm_overlay because this member
  isn't used anymore.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/exynos/exynos_drm_fbdev.c

@@ -393,6 +393,10 @@ int exynos_drm_fbdev_reinit(struct drm_device *dev)
 	if (!private)
 		return -EINVAL;
 
+	/*
+	 * if all sub drivers were unloaded then num_connector is 0
+	 * so at this time, the framebuffers also should be destroyed.
+	 */
 	if (!dev->mode_config.num_connector) {
 		exynos_drm_fbdev_fini(dev);
 		return 0;
@@ -429,7 +433,7 @@ int exynos_drm_fbdev_reinit(struct drm_device *dev)
 		 * re-configure the fb helper. it means that this function
 		 * has been called by the specific drivers.
 		 */
-		return exynos_drm_fbdev_init(dev);
+		ret = exynos_drm_fbdev_init(dev);
 	}
 
 	return ret;