drm/panel: otm8009a: Add support for the optional power-supply

Add support for the optional power-supply.

Note: A "dummy regulator" is returned by devm_regulator_get()
if the optional regulator is not present in the device tree,
simplifying the source code when enabling/disabling the regulator.

Signed-off-by: Philippe Cornu <[e-mail]>
Reviewed-by: Yannick Fertré <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/panel/panel-orisetech-otm8009a.c

···
 #include <drm/drm_panel.h>
 #include <linux/backlight.h>
 #include <linux/gpio/consumer.h>
+#include <linux/regulator/consumer.h>
 #include <video/mipi_display.h>
 
 #define DRV_NAME "orisetech_otm8009a"
···
 	struct drm_panel panel;
 	struct backlight_device *bl_dev;
 	struct gpio_desc *reset_gpio;
+	struct regulator *supply;
 	bool prepared;
 	bool enabled;
 };
···
 		msleep(20);
 	}
 
+	regulator_disable(ctx->supply);
+
 	ctx->prepared = false;
 
 	return 0;
···
 
 	if (ctx->prepared)
 		return 0;
+
+	ret = regulator_enable(ctx->supply);
+	if (ret < 0) {
+		DRM_ERROR("failed to enable supply: %d\n", ret);
+		return ret;
+	}
 
 	if (ctx->reset_gpio) {
 		gpiod_set_value_cansleep(ctx->reset_gpio, 0);
···
 	if (IS_ERR(ctx->reset_gpio)) {
 		dev_err(dev, "cannot get reset-gpio\n");
 		return PTR_ERR(ctx->reset_gpio);
+	}
+
+	ctx->supply = devm_regulator_get(dev, "power");
+	if (IS_ERR(ctx->supply)) {
+		ret = PTR_ERR(ctx->supply);
+		dev_err(dev, "failed to request regulator: %d\n", ret);
+		return ret;
 	}
 
 	mipi_dsi_set_drvdata(dsi, ctx);