drm/rockchip: Fix suspend crash when drm is not bound

Currently we are allocating drm_device in rockchip_drm_bind, so if the
suspend/resume code access it when drm is not bound, we would hit this
crash:

[ 253.402836] Unable to handle kernel NULL pointer dereference at virtual address 00000028
[ 253.402837] pgd = ffffffc06c9b0000
[ 253.402841] [00000028] *pgd=0000000000000000, *pud=0000000000000000
[ 253.402844] Internal error: Oops: 96000005 [#1] PREEMPT SMP
[ 253.402859] Modules linked in: btusb btrtl btbcm btintel bluetooth ath10k_pci ath10k_core ar10k_ath ar10k_mac80211 cfg80211 ip6table_filter asix usbnet mii
[ 253.402864] CPU: 4 PID: 1331 Comm: cat Not tainted 4.4.70 #15
[ 253.402865] Hardware name: Google Scarlet (DT)
[ 253.402867] task: ffffffc076c0ce00 ti: ffffffc06c2c8000 task.ti: ffffffc06c2c8000
[ 253.402871] PC is at rockchip_drm_sys_suspend+0x20/0x5c

Add sanity checks to prevent that.

Reported-by: Brian Norris <[e-mail]>
Signed-off-by: Jeffy Chen <[e-mail]>
Signed-off-by: Sean Paul <[e-mail]>
Link: https://patchwork.kernel.org/patch/9890297/

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

···
 static int rockchip_drm_sys_suspend(struct device *dev)
 {
 	struct drm_device *drm = dev_get_drvdata(dev);
-	struct rockchip_drm_private *priv = drm->dev_private;
+	struct rockchip_drm_private *priv;
+
+	if (!drm)
+		return 0;
 
 	drm_kms_helper_poll_disable(drm);
 	rockchip_drm_fb_suspend(drm);
 
+	priv = drm->dev_private;
 	priv->state = drm_atomic_helper_suspend(drm);
 	if (IS_ERR(priv->state)) {
 		rockchip_drm_fb_resume(drm);
···
 static int rockchip_drm_sys_resume(struct device *dev)
 {
 	struct drm_device *drm = dev_get_drvdata(dev);
-	struct rockchip_drm_private *priv = drm->dev_private;
+	struct rockchip_drm_private *priv;
 
+	if (!drm)
+		return 0;
+
+	priv = drm->dev_private;
 	drm_atomic_helper_resume(drm, priv->state);
 	rockchip_drm_fb_resume(drm);
 	drm_kms_helper_poll_enable(drm);