drm/panfrost: Enable devfreq to work without regulator

If there is no regulator defined for the GPU then still control the
frequency using the supplied clock.

Some boards have clock control but no (direct) control of the regulator.
For example the HiKey960 uses a mailbox protocol to a MCU to control
frequencies and doesn't directly control the voltage. This patch allows
frequency control of the GPU on this system.
Signed-off-by: Steven Price <steven.price@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816093107.30518-1-steven.price@arm.com

drivers/gpu/drm/panfrost/panfrost_devfreq.c

@@ -39,7 +39,7 @@ static int panfrost_devfreq_target(struct device *dev, unsigned long *freq,
 	 * If frequency scaling from low to high, adjust voltage first.
 	 * If frequency scaling from high to low, adjust frequency first.
 	 */
-	if (old_clk_rate < target_rate) {
+	if (old_clk_rate < target_rate && pfdev->regulator) {
 		err = regulator_set_voltage(pfdev->regulator, target_volt,
 					    target_volt);
 		if (err) {
@@ -58,7 +58,7 @@ static int panfrost_devfreq_target(struct device *dev, unsigned long *freq,
 		return err;
 	}
 
-	if (old_clk_rate > target_rate) {
+	if (old_clk_rate > target_rate && pfdev->regulator) {
 		err = regulator_set_voltage(pfdev->regulator, target_volt,
 					    target_volt);
 		if (err)
@@ -136,9 +136,6 @@ int panfrost_devfreq_init(struct panfrost_device *pfdev)
 	int ret;
 	struct dev_pm_opp *opp;
 
-	if (!pfdev->regulator)
-		return 0;
-
 	ret = dev_pm_opp_of_add_table(&pfdev->pdev->dev);
 	if (ret == -ENODEV) /* Optional, continue without devfreq */
 		return 0;