drm/panfrost: make devfreq optional again

Devfreq runtime usage was made mandatory, thus making panfrost fail to probe
on Amlogic S912 SoCs missing the "operating-points-v2" property.
Make it optional again, leaving PM_DEVFREQ selected by default.

Fixes: f3617b449d0b ("drm/panfrost: Select devfreq")
Signed-off-by: Neil Armstrong <[e-mail]>
Reviewed-by: Ezequiel Garcia <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panfrost/panfrost_devfreq.c

···
 		return 0;
 
 	ret = dev_pm_opp_of_add_table(&pfdev->pdev->dev);
-	if (ret)
+	if (ret == -ENODEV) /* Optional, continue without devfreq */
+		return 0;
+	else if (ret)
 		return ret;
 
 	panfrost_devfreq_reset(pfdev);
···
 {
 	int i;
 
+	if (!pfdev->devfreq.devfreq)
+		return;
+
 	panfrost_devfreq_reset(pfdev);
 	for (i = 0; i < NUM_JOB_SLOTS; i++)
 		pfdev->devfreq.slot[i].busy = false;
···
 
 void panfrost_devfreq_suspend(struct panfrost_device *pfdev)
 {
+	if (!pfdev->devfreq.devfreq)
+		return;
+
 	devfreq_suspend_device(pfdev->devfreq.devfreq);
 }
 
···
 	struct panfrost_devfreq_slot *devfreq_slot = &pfdev->devfreq.slot[slot];
 	ktime_t now;
 	ktime_t last;
+
+	if (!pfdev->devfreq.devfreq)
+		return;
 
 	now = ktime_get();
 	last = pfdev->devfreq.slot[slot].time_last_update;